flash_fwd.py

# Copyright (c) 2025, Jay Shah, Ganesh Bikshandi, Ying Zhang, Vijay Thakkar, Pradeep Ramani, Tri Dao.
# A reimplementation of
# https://github.com/Dao-AILab/flash-attention/blob/main/hopper/flash_fwd_kernel_sm80.h
# and https://github.com/Dao-AILab/flash-attention/blob/main/hopper/flash_fwd_kernel_sm90.h
# from Cutlass C++ to Cute-DSL.
# Built on Cute-DSL example: https://github.com/NVIDIA/cutlass/blob/main/examples/python/CuTeDSL/ampere/flash_attention_v2.py

import math
from types import SimpleNamespace
from typing import Type, Callable, Optional
from functools import partial

import cuda.bindings.driver as cuda

import cutlass
import cutlass.cute as cute
from cutlass import Float32, Int32, const_expr
from cutlass.cute.nvgpu import cpasync, warp
import cutlass.utils as utils_basic
from cutlass.base_dsl.arch import Arch
from cutlass.cutlass_dsl import BaseDSL

from quack import copy_utils
from quack import layout_utils

from flash_attn.cute import ampere_helpers as sm80_utils
from flash_attn.cute.cute_dsl_utils import assume_tensor_aligned
from flash_attn.cute import utils
from flash_attn.cute.mask import AttentionMask
from flash_attn.cute.softmax import Softmax, apply_score_mod_inner
from flash_attn.cute.seqlen_info import SeqlenInfoQK
from flash_attn.cute.block_info import BlockInfo
from flash_attn.cute.pack_gqa import PackGQA, pack_gqa_layout
from flash_attn.cute.named_barrier import NamedBarrierFwd
from flash_attn.cute.block_sparsity import BlockSparseTensors
from flash_attn.cute.tile_scheduler import SingleTileScheduler, SingleTileVarlenScheduler, TileSchedulerArguments
from flash_attn.cute.utils import AuxData


class FlashAttentionForwardBase:

    def __init__(
        self,
        dtype: Type[cutlass.Numeric],
        head_dim: int,
        head_dim_v: Optional[int] = None,
        qhead_per_kvhead: int = 1,
        is_causal: bool = False,
        is_local: bool = False,
        pack_gqa: bool = True,
        tile_m: int = 128,
        tile_n: int = 128,
        num_stages: int = 1,
        num_threads: int = 128,
        Q_in_regs: bool = False,
        score_mod: Optional[cutlass.Constexpr] = None,
        mask_mod: Optional[cutlass.Constexpr] = None,
        has_aux_tensors: bool = False,
        q_subtile_factor: int = 1,
    ):
        """Initializes the configuration for a flash attention kernel.

        All contiguous dimensions must be at least 16 bytes aligned, which means that the head dimension
        should be a multiple of 8.

        :param head_dim: head dimension
        :type head_dim: int
        :param tile_m: m block size
        :type tile_m: int
        :param tile_n: n block size
        :type tile_n: int
        :param num_threads: number of threads
        :type num_threads: int
        :param is_causal: is causal
        :param score_mod: A callable that takes the attention scores and applies a modification.
            Callable signature: ``score_mod(scores, batch_idx, head_idx, q_idx, kv_idx, aux_tensors) -> Any``
        :param mask_mod: A callable that takes the attention scores and returns a boolean representing whether that score should be masked.
            Callable signature: ``mask_mod(batch_idx, head_idx, q_idx, kv_idx, aux_tensors) -> Boolean``
        """
        self.dtype = dtype
        # padding head_dim to a multiple of 16 as k_block_size
        hdim_multiple_of = 16
        self.tile_hdim = int(math.ceil(head_dim / hdim_multiple_of) * hdim_multiple_of)
        head_dim_v = head_dim_v if head_dim_v is not None else head_dim
        self.same_hdim_kv = head_dim == head_dim_v
        self.tile_hdimv = int(math.ceil(head_dim_v / hdim_multiple_of) * hdim_multiple_of)
        # Can save registers (and hence be faster) if we don't have to check hdim predication
        self.check_hdim_oob = head_dim != self.tile_hdim
        self.check_hdim_v_oob = head_dim_v != self.tile_hdimv
        self.qhead_per_kvhead = qhead_per_kvhead
        self.is_causal = is_causal
        self.is_local = is_local
        self.pack_gqa = pack_gqa
        self.tile_m = tile_m
        self.tile_n = tile_n
        self.num_threads = num_threads
        self.num_stages = num_stages
        self.q_subtile_factor = q_subtile_factor
        self.Q_in_regs = Q_in_regs
        self.score_mod = score_mod
        self.mask_mod = mask_mod
        self.qk_acc_dtype = Float32
        self.score_vec_size: cutlass.Constexpr = getattr(
            score_mod, "__vec_size__", 1 if cutlass.const_expr(has_aux_tensors) else 2
        )
        if self.score_vec_size > 2:
            raise ValueError(
                f"score_mod vec_size {self.score_vec_size} not supported on Sm80/90/120 "
                "due to accumulator thread ownership pattern."
            )
        self.mask_vec_size: cutlass.Constexpr = getattr(mask_mod, "__vec_size__", 1)
        if self.mask_vec_size > 1:
            raise ValueError(
                f"mask_mod vec_size {self.mask_vec_size} not supported on Sm80/90/120 "
                "due to accumulator thread ownership pattern."
            )
        self.arch = BaseDSL._get_dsl().get_arch_enum()

    @staticmethod
    def can_implement(
        dtype,
        head_dim,
        head_dim_v,
        tile_m,
        tile_n,
        num_stages,
        num_threads,
        is_causal,
        Q_in_regs=False,
    ) -> bool:
        """Check if the kernel can be implemented with the given parameters.

        :param dtype: data type
        :type dtype: cutlass.Numeric
        :param head_dim: head dimension
        :type head_dim: int
        :param tile_m: m block size
        :type tile_m: int
        :param tile_n: n block size
        :type tile_n: int
        :param num_threads: number of threads
        :type num_threads: int
        :param is_causal: is causal
        :type is_causal: bool

        :return: True if the kernel can be implemented, False otherwise
        :rtype: bool
        """
        if dtype not in [cutlass.Float16, cutlass.BFloat16]:
            return False
        if head_dim % 8 != 0:
            return False
        if head_dim_v % 8 != 0:
            return False
        if tile_n % 16 != 0:
            return False
        if num_threads % 32 != 0:
            return False
        # Check if block size setting is out of shared memory capacity
        # Shared memory usage: Q tile + (K tile + V tile) where K and V use the same tile size
        smem_usage_Q = tile_m * head_dim * 2
        smem_usage_K = tile_n * head_dim * num_stages * 2
        smem_usage_V = tile_n * head_dim_v * num_stages * 2
        smem_usage_QV = (
            (smem_usage_Q + smem_usage_V) if not Q_in_regs else max(smem_usage_Q, smem_usage_V)
        )
        smem_usage = smem_usage_QV + smem_usage_K
        # TODO: sm86 and sm89
        smem_capacity = utils_basic.get_smem_capacity_in_bytes("sm_80")
        if smem_usage > smem_capacity:
            return False
        # Check if twice the block size is divisible by the number of threads
        if (tile_m * 2) % num_threads != 0:
            return False
        return True

    def _check_type(
        self,
        mQ_type: Type[cutlass.Numeric],
        mK_type: Type[cutlass.Numeric],
        mV_type: Type[cutlass.Numeric],
        mO_type: Type[cutlass.Numeric],
        mLSE_type: Type[cutlass.Numeric] | None,
        mCuSeqlensQ_type: Type[cutlass.Numeric] | None,
        mCuSeqlensK_type: Type[cutlass.Numeric] | None,
        mSeqUsedQ_type: Type[cutlass.Numeric] | None,
        mSeqUsedK_type: Type[cutlass.Numeric] | None,
    ):
        # Get the data type and check if it is fp16 or bf16
        if const_expr(not (mQ_type == mK_type == mV_type == mO_type)):
            raise TypeError("All tensors must have the same data type")
        if const_expr(mQ_type not in [cutlass.Float16, cutlass.BFloat16]):
            raise TypeError("Only Float16 or BFloat16 is supported")
        if const_expr(mLSE_type not in [None, Float32]):
            raise TypeError("LSE tensor must be Float32")
        if const_expr(mCuSeqlensQ_type not in [None, Int32]):
            raise TypeError("cu_seqlens_q tensor must be Int32")
        if const_expr(mCuSeqlensK_type not in [None, Int32]):
            raise TypeError("cu_seqlens_k tensor must be Int32")
        if const_expr(mSeqUsedQ_type not in [None, Int32]):
            raise TypeError("seqused_q tensor must be Int32")
        if const_expr(mSeqUsedK_type not in [None, Int32]):
            raise TypeError("seqused_k tensor must be Int32")
        assert mQ_type == self.dtype

    def _setup_attributes(self):
        # ///////////////////////////////////////////////////////////////////////////////
        # Shared memory layout: Q/K/V
        # ///////////////////////////////////////////////////////////////////////////////
        sQ_layout_atom, sK_layout_atom, sV_layout_atom, sO_layout_atom, sP_layout_atom = (
            self._get_smem_layout_atom()
        )
        self.sQ_layout = cute.tile_to_shape(
            sQ_layout_atom,
            (self.tile_m, self.tile_hdim),
            (0, 1),
        )
        self.sK_layout = cute.tile_to_shape(
            sK_layout_atom,
            (self.tile_n, self.tile_hdim, self.num_stages),
            (0, 1, 2),
        )
        self.sV_layout = cute.tile_to_shape(
            sV_layout_atom,
            (self.tile_n, self.tile_hdimv, self.num_stages),
            (0, 1, 2),
        )
        self.sO_layout = cute.tile_to_shape(
            sO_layout_atom,
            (self.tile_m, self.tile_hdimv),
            (0, 1),
        )
        if const_expr(sP_layout_atom is not None):
            self.sP_layout = cute.tile_to_shape(
                sP_layout_atom,
                (self.tile_m, self.tile_n),
                (0, 1),
            )
        else:
            self.sP_layout = None

        # ///////////////////////////////////////////////////////////////////////////////
        # GMEM Tiled copy:
        # ///////////////////////////////////////////////////////////////////////////////
        # Thread layouts for copies
        universal_copy_bits = 128
        async_copy_elems = universal_copy_bits // self.dtype.width
        # atom_async_copy: async copy atom for QKV load
        atom_async_copy = cute.make_copy_atom(
            cpasync.CopyG2SOp(cache_mode=cpasync.LoadCacheMode.GLOBAL),
            self.dtype,
            num_bits_per_copy=universal_copy_bits,
        )
        # atom_universal_copy: universal copy atom for O store
        atom_universal_copy = cute.make_copy_atom(
            cute.nvgpu.CopyUniversalOp(),
            self.dtype,
            num_bits_per_copy=universal_copy_bits,
        )
        # tQ_layout and tK_layout: thread layout for QK load
        tQK_shape_dim_1 = sQ_layout_atom.outer.shape[1] // async_copy_elems
        assert self.num_Q_load_threads % tQK_shape_dim_1 == 0, (
            "num_threads must be divisible by tQK_shape_dim_1"
        )
        assert self.num_producer_threads % tQK_shape_dim_1 == 0, (
            "num_threads must be divisible by tQK_shape_dim_1"
        )
        tQ_layout = cute.make_ordered_layout(
            (self.num_Q_load_threads // tQK_shape_dim_1, tQK_shape_dim_1),
            order=(1, 0),
        )
        tK_layout = cute.make_ordered_layout(
            (self.num_producer_threads // tQK_shape_dim_1, tQK_shape_dim_1),
            order=(1, 0),
        )
        # So that we don't have to check if we overshoot kBlockM when we load Q
        assert self.tile_m % tQ_layout.shape[0] == 0
        tV_shape_dim_1 = sV_layout_atom.outer.shape[1] // async_copy_elems
        tV_layout = cute.make_ordered_layout(
            (self.num_producer_threads // tV_shape_dim_1, tV_shape_dim_1),
            order=(1, 0),
        )
        # TODO: need a different layout for O if O dtype is not the same as V dtype
        # tO_layout: thread layout for O store
        tO_layout = cute.make_ordered_layout(
            (self.num_epilogue_threads // tV_shape_dim_1, tV_shape_dim_1),
            order=(1, 0),
        )
        # So that we don't have to check if we overshoot kBlockM when we store O
        assert self.tile_m % tO_layout.shape[0] == 0

        # Value layouts for copies
        vQKV_layout = cute.make_layout((1, async_copy_elems))
        vO_layout = vQKV_layout

        self.gmem_tiled_copy_Q = cute.make_tiled_copy_tv(atom_async_copy, tQ_layout, vQKV_layout)
        self.gmem_tiled_copy_K = cute.make_tiled_copy_tv(atom_async_copy, tK_layout, vQKV_layout)
        self.gmem_tiled_copy_V = cute.make_tiled_copy_tv(atom_async_copy, tV_layout, vQKV_layout)
        # gmem_tiled_copy_O: tiled copy for O store
        self.gmem_tiled_copy_O = cute.make_tiled_copy_tv(atom_universal_copy, tO_layout, vO_layout)

    def _get_smem_layout_atom(self):
        raise NotImplementedError()

    def _get_tiled_mma(self):
        raise NotImplementedError()

    def _get_shared_storage_cls(self):
        raise NotImplementedError()

    @cute.jit
    def __call__(
        self,
        mQ: cute.Tensor,
        mK: cute.Tensor,
        mV: cute.Tensor,
        mO: cute.Tensor,
        mLSE: Optional[cute.Tensor],
        softmax_scale: Float32,
        # Always keep stream as the last parameter (EnvStream: obtained implicitly via TVM FFI).
        stream: cuda.CUstream = None,
    ):
        """Configures and launches the flash attention kernel.

        mQ/mK/mV/mO has same data types(supports fp16 and bf16) and same layout:
        (batch_size, seqlen_q, num_head, head_dim):(_, _, _, 1)
        """
        raise NotImplementedError()

    @cute.jit
    def epilogue(
        self,
        acc_O: cute.Tensor,
        lse: cute.Tensor,
        mO: cute.Tensor,
        mLSE: Optional[cute.Tensor],
        sO: cute.Tensor,
        seqlen: SeqlenInfoQK,
        gmem_tiled_copy_O: cute.TiledCopy,
        tma_atom_O: Optional[cute.CopyAtom],
        tiled_mma: cute.TiledMma,
        tidx: Int32,
        m_block: Int32,
        head_idx: Int32,
        batch_idx: Int32,
    ):
        # store acc_O
        rO = cute.make_fragment_like(acc_O, self.dtype)
        rO.store(acc_O.load().to(self.dtype))
        # Make sure all threads have finished reading V
        cute.arch.barrier(
            barrier_id=int(NamedBarrierFwd.Epilogue), number_of_threads=self.num_epilogue_threads
        )
        smem_copy_atom_O = utils.get_smem_store_atom(self.arch.major * 10 + self.arch.minor, self.dtype)
        smem_thr_copy_O = cute.make_tiled_copy_C(smem_copy_atom_O, tiled_mma).get_slice(tidx)
        taccOrO = smem_thr_copy_O.retile(rO)
        taccOsO = smem_thr_copy_O.partition_D(sO)
        # taccOsO = copy_utils.partition_D_position_independent(smem_thr_copy_O, sO)
        # copy acc O from rmem to smem with the smem copy atom
        cute.copy(smem_copy_atom_O, taccOrO, taccOsO)

        cO = cute.make_identity_tensor((self.tile_m, self.tile_hdimv))
        pack_gqa = PackGQA(
            self.tile_m, self.tile_hdimv, self.check_hdim_v_oob, self.qhead_per_kvhead
        )

        # Write LSE from rmem -> gmem
        if const_expr(mLSE is not None):
            mLSE_cur = seqlen.offset_batch_Q(mLSE, batch_idx, dim=2)[None, head_idx]
            if const_expr(not self.pack_gqa):
                gLSE = cute.local_tile(mLSE_cur, (self.tile_m,), (m_block,))
                gLSE_expanded_layout = cute.append(
                    gLSE.layout, cute.make_layout((self.tile_hdimv,), stride=(0,))
                )
                gLSE_expanded = cute.make_tensor(gLSE.iterator, gLSE_expanded_layout)
                thr_mma = tiled_mma.get_slice(tidx)
                taccOgLSE = layout_utils.reshape_acc_to_mn(thr_mma.partition_C(gLSE_expanded))
                assert cute.size(taccOgLSE, mode=[0]) == cute.size(lse)
                taccOcO = layout_utils.reshape_acc_to_mn(thr_mma.partition_C(cO))
                t0accOcO = layout_utils.reshape_acc_to_mn(thr_mma.get_slice(0).partition_C(cO))
                # Only the thread corresponding to column 0 writes out the lse to gmem
                if taccOcO[0][1] == 0:
                    for m in cutlass.range(cute.size(taccOgLSE.shape[1]), unroll_full=True):
                        if (
                            t0accOcO[m, 0][0]
                            < seqlen.seqlen_q - m_block * self.tile_m - taccOcO[0][0]
                        ):
                            taccOgLSE[m, 0] = lse[m]
            else:
                pack_gqa.store_LSE(mLSE_cur, lse, tiled_mma, tidx, m_block, seqlen.seqlen_q)

        ragged = self.use_tma_O and (seqlen.has_cu_seqlens_q or seqlen.has_seqused_q)
        mO_cur = seqlen.offset_batch_Q(mO, batch_idx, dim=3, ragged=ragged)[None, None, head_idx]
        # thr_mma = tiled_mma.get_slice(tidx)
        # taccOgO = thr_mma.partition_C(gO)
        # cute.autovec_copy(rO, taccOgO)
        # sync to make sure all smem stores are done
        if const_expr(self.use_tma_O):
            # ensure smem writes are visible to TMA
            cute.arch.fence_view_async_shared()
            cute.arch.barrier_arrive(
                barrier_id=int(NamedBarrierFwd.Epilogue),
                number_of_threads=self.num_epilogue_threads + cute.arch.WARP_SIZE,
            )
            gO = cute.local_tile(mO_cur, (self.tile_m, self.tile_hdimv), (m_block, 0))
            store_O, _, _ = copy_utils.tma_get_copy_fn(
                tma_atom_O, 0, cute.make_layout(1), sO, gO, single_stage=True
            )
            warp_idx = cute.arch.make_warp_uniform(cute.arch.warp_idx())
            if warp_idx == 4:
                cute.arch.barrier(
                    barrier_id=int(NamedBarrierFwd.Epilogue),
                    number_of_threads=self.num_epilogue_threads + cute.arch.WARP_SIZE,
                )
                store_O()
                cute.arch.cp_async_bulk_commit_group()
                cute.arch.cp_async_bulk_wait_group(0, read=True)
        else:
            cute.arch.barrier(
                barrier_id=int(NamedBarrierFwd.Epilogue),
                number_of_threads=self.num_epilogue_threads,
            )
            gmem_thr_copy_O = gmem_tiled_copy_O.get_slice(tidx)
            tOsO = gmem_thr_copy_O.partition_S(sO)
            tOrO = cute.make_fragment_like(tOsO, self.dtype)
            # load acc O from smem to rmem for wider vectorization
            cute.autovec_copy(tOsO, tOrO)
            if const_expr(not self.pack_gqa):
                gO = cute.local_tile(mO_cur, (self.tile_m, self.tile_hdimv), (m_block, 0))
                tOgO = gmem_thr_copy_O.partition_D(gO)
                tOcO = gmem_thr_copy_O.partition_S(cO)
                t0OcO = gmem_tiled_copy_O.get_slice(0).partition_S(cO)
                tOpO = utils.predicate_k(tOcO, limit=mO.shape[1])
                # copy acc O from rmem to gmem
                for rest_m in cutlass.range_constexpr(cute.size(tOrO.shape[1])):
                    if (
                        t0OcO[0, rest_m, 0][0]
                        < seqlen.seqlen_q - m_block * self.tile_m - tOcO[0][0]
                    ):
                        cute.copy(
                            gmem_tiled_copy_O,
                            tOrO[None, rest_m, None],
                            tOgO[None, rest_m, None],
                            pred=tOpO[None, rest_m, None]
                            if const_expr(self.check_hdim_v_oob)
                            else None,
                        )
            else:
                pack_gqa.store_O(mO_cur, tOrO, gmem_tiled_copy_O, tidx, m_block, seqlen.seqlen_q)

    @cute.jit
    def advance_pipeline(self, pipeline_index):
        return pipeline_index + 1 if pipeline_index < self.num_stages - 1 else 0

    @cute.jit
    def load_Q(
        self,
        gmem_thr_copy: cute.TiledCopy,
        gQ: cute.Tensor,
        sQ: cute.Tensor,
        block: Int32,
        seqlen: Int32,
        headdim: Int32,
    ):
        tQsQ, tQgQ = gmem_thr_copy.partition_D(sQ), gmem_thr_copy.partition_S(gQ)
        cQ = cute.make_identity_tensor((self.tile_m, self.tile_hdim))
        tQcQ = gmem_thr_copy.partition_S(cQ)
        t0QcQ = gmem_thr_copy.get_slice(0).partition_S(cQ)
        tQpQ = utils.predicate_k(tQcQ, limit=headdim)
        for m in cutlass.range_constexpr(cute.size(tQsQ.shape[1])):
            # Instead of using tQcQ, we using t0QcQ and subtract the offset from the limit
            # (seqlen - block * kBlockM). This is because the entries of t0QcQ are known at compile time.
            if t0QcQ[0, m, 0][0] < seqlen - block * self.tile_m - tQcQ[0][0]:
                cute.copy(
                    gmem_thr_copy,
                    tQgQ[None, m, None],
                    tQsQ[None, m, None],
                    pred=tQpQ[None, m, None] if const_expr(self.check_hdim_oob) else None,
                )
            # We don't need to clear the sQ smem tiles since we'll only write out the valid outputs

    @cute.jit
    def load_K(
        self,
        gmem_tiled_copy: cute.TiledCopy,
        tKgK: cute.Tensor,
        tKsK: cute.Tensor,
        tKcK: cute.Tensor,
        t0KcK: cute.Tensor,
        tKpK: cute.Tensor,
        block: Int32,
        smem_pipe_write: Int32,
        seqlen: Int32,
        need_predicates: cutlass.Constexpr,
    ):
        # Do we need to check if we overshoot kBlockN when we load K?
        is_even_n_smem_k = self.tile_n % gmem_tiled_copy.tiler_mn[0].shape == 0
        if const_expr(need_predicates or not is_even_n_smem_k):
            # Instead of using tKcK, we using t0KcK and subtract the offset from the limit
            # (seqlen - block * kBlockN). This is because the entries of t0KcK are known at compile time.
            if const_expr(is_even_n_smem_k):
                seqlen_limit = seqlen - block * self.tile_n
            else:
                if const_expr(not need_predicates):
                    seqlen_limit = self.tile_n
                else:
                    seqlen_limit = cutlass.min(seqlen - block * self.tile_n, self.tile_n)
            seqlen_limit -= tKcK[0][0]
            for n in cutlass.range_constexpr(cute.size(tKsK.shape[1])):
                if t0KcK[0, n, 0][0] < seqlen_limit:
                    cute.copy(
                        gmem_tiled_copy,
                        tKgK[None, n, None, block],
                        tKsK[
                            None, n, None, smem_pipe_write if const_expr(self.num_stages > 1) else 0
                        ],
                        pred=tKpK[None, n, None] if const_expr(self.check_hdim_oob) else None,
                    )
                # We don't need to clear the sK smem tiles since we'll mask out the scores anyway.
        else:
            cute.copy(
                gmem_tiled_copy,
                tKgK[None, None, None, block],
                tKsK[None, None, None, smem_pipe_write if const_expr(self.num_stages > 1) else 0],
                pred=tKpK if const_expr(self.check_hdim_oob) else None,
            )

    @cute.jit
    def load_V(
        self,
        gmem_tiled_copy: cute.TiledCopy,
        tVgV: cute.Tensor,
        tVsV: cute.Tensor,
        tVcV: cute.Tensor,
        t0VcV: cute.Tensor,
        tVpV: cute.Tensor,
        block: Int32,
        smem_pipe_write: Int32,
        seqlen: Int32,
        need_predicates: cutlass.Constexpr,
    ):
        # Do we need to check if we overshoot kBlockN when we load V?
        is_even_n_smem_v = self.tile_n % gmem_tiled_copy.tiler_mn[0].shape == 0
        if const_expr(need_predicates or not is_even_n_smem_v):
            for n in cutlass.range_constexpr(cute.size(tVsV.shape[1])):
                # If kBlockN doesn't evenly divide the tiled copy, only the last `n` needs to be checked
                if (
                    is_even_n_smem_v
                    or n < cute.size(tVsV.shape[1]) - 1
                    or tVcV[0, n, 0][0] < self.tile_n
                ):
                    predicate = tVpV[None, n, None] if const_expr(self.check_hdim_v_oob) else None
                    if const_expr(need_predicates):
                        seqlen_limit = seqlen - block * self.tile_n - tVcV[0][0]
                        predicate_n = t0VcV[0, n, 0][0] < seqlen_limit
                        predicate = cute.make_fragment_like(tVpV[None, 0, None])
                        for k in cutlass.range_constexpr(cute.size(predicate.shape[1])):
                            for i in cutlass.range_constexpr(cute.size(predicate.shape[0])):
                                predicate[i, k] = (
                                    tVpV[i, n, k] if const_expr(self.check_hdim_v_oob) else True
                                ) and predicate_n
                    cute.copy(
                        gmem_tiled_copy,
                        tVgV[None, n, None, block],
                        tVsV[
                            None, n, None, smem_pipe_write if const_expr(self.num_stages > 1) else 0
                        ],
                        pred=predicate,
                    )
        else:
            cute.copy(
                gmem_tiled_copy,
                tVgV[None, None, None, block],
                tVsV[None, None, None, smem_pipe_write if const_expr(self.num_stages > 1) else 0],
                pred=tVpV if const_expr(self.check_hdim_v_oob) else None,
            )


class FlashAttentionForwardSm80(FlashAttentionForwardBase):
    def _get_smem_layout_atom(self):
        sQ_layout_atom = sm80_utils.get_smem_layout_atom(self.dtype, self.tile_hdim)
        sK_layout_atom = sQ_layout_atom
        sV_layout_atom = sm80_utils.get_smem_layout_atom(self.dtype, self.tile_hdimv)
        sO_layout_atom = sV_layout_atom
        sP_layout_atom = None
        return sQ_layout_atom, sK_layout_atom, sV_layout_atom, sO_layout_atom, sP_layout_atom

    def _get_tiled_mma(self):
        tiled_mma_qk = cute.make_tiled_mma(
            warp.MmaF16BF16Op(self.dtype, Float32, (16, 8, 16)),
            (self.num_threads // 32, 1, 1),
            permutation_mnk=(self.num_threads // 32 * 16, 16, 16),
        )
        tiled_mma_pv = cute.make_tiled_mma(
            warp.MmaF16BF16Op(self.dtype, Float32, (16, 8, 16)),
            (self.num_threads // 32, 1, 1),
            permutation_mnk=(self.num_threads // 32 * 16, 16, 16),
        )
        return tiled_mma_qk, tiled_mma_pv

    def _get_shared_storage_cls(self):
        sQ_struct, sK_struct, sV_struct = [
            cute.struct.Align[cute.struct.MemRange[self.dtype, cute.cosize(layout)], 1024]
            for layout in (self.sQ_layout, self.sK_layout, self.sV_layout)
        ]
        cosize_sQV = max(cute.cosize(self.sQ_layout), cute.cosize(self.sV_layout))
        sQV_struct = cute.struct.Align[cute.struct.MemRange[self.dtype, cosize_sQV], 1024]

        @cute.struct
        class SharedStorageQKV:
            sV: sV_struct
            sQ: sQ_struct
            sK: sK_struct

        @cute.struct
        class SharedStorageSharedQV:
            sQ: sQV_struct
            sK: sK_struct

        return SharedStorageQKV if const_expr(not self.Q_in_regs) else SharedStorageSharedQV

    @cute.jit
    def __call__(
        self,
        mQ: cute.Tensor,
        mK: cute.Tensor,
        mV: cute.Tensor,
        mO: cute.Tensor,
        mLSE: Optional[cute.Tensor],
        softmax_scale: Float32,
        mCuSeqlensQ: Optional[cute.Tensor] = None,
        mCuSeqlensK: Optional[cute.Tensor] = None,
        mSeqUsedQ: Optional[cute.Tensor] = None,
        mSeqUsedK: Optional[cute.Tensor] = None,
        mPageTable: Optional[cute.Tensor] = None,
        window_size_left: Int32 | int | None = None,
        window_size_right: Int32 | int | None = None,
        learnable_sink: Optional[cute.Tensor] = None,
        blocksparse_tensors: Optional[BlockSparseTensors] = None,
        aux_data: AuxData = AuxData(),
        mCuTotalMBlocks: Optional[cute.Tensor] = None,
        mCuTotalSplitsMBlocks: Optional[cute.Tensor] = None,
        # Always keep stream as the last parameter (EnvStream: obtained implicitly via TVM FFI).
        stream: cuda.CUstream = None,
    ):
        """Configures and launches the flash attention kernel.

        mQ/mK/mV/mO has same data types(supports fp16 and bf16) and same layout:
        (batch_size, seqlen_q, num_head, head_dim):(_, _, _, 1)
        """
        assert learnable_sink is None, "Learnable sink is not supported in this kernel"
        self._check_type(
            *(t.element_type if t is not None else None for t in (mQ, mK, mV, mO, mLSE, mCuSeqlensQ, mCuSeqlensK, mSeqUsedQ, mSeqUsedK))
        )
        tiled_mma_qk, tiled_mma_pv = self._get_tiled_mma()
        self.num_mma_threads = tiled_mma_pv.size
        self.num_producer_threads = self.num_threads
        self.num_Q_load_threads = self.num_threads
        self.num_epilogue_threads = self.num_threads
        self.use_tma_O = Arch.sm_90 <= self.arch < Arch.sm_120
        self._setup_attributes()
        SharedStorage = self._get_shared_storage_cls()
        mQ, mK, mV, mO = [assume_tensor_aligned(t) for t in (mQ, mK, mV, mO)]
        # Layout permutation: 4D non-varlen vs 3D varlen
        QO_layout_transpose = [1, 3, 2, 0] if const_expr(mCuSeqlensQ is None) else [0, 2, 1]
        KV_layout_transpose = [1, 3, 2, 0] if const_expr(mCuSeqlensK is None) else [0, 2, 1]
        mQ, mO = [
            cute.make_tensor(t.iterator, cute.select(t.layout, mode=QO_layout_transpose))
            for t in (mQ, mO)
        ]
        mK, mV = [
            cute.make_tensor(t.iterator, cute.select(t.layout, mode=KV_layout_transpose))
            for t in (mK, mV)
        ]
        if const_expr(mLSE is not None):
            LSE_layout_transpose = [2, 1, 0] if const_expr(mCuSeqlensQ is None) else [1, 0]
            mLSE = cute.make_tensor(mLSE.iterator, cute.select(mLSE.layout, mode=LSE_layout_transpose))
        if const_expr(self.pack_gqa):
            nheads_kv = mK.shape[2]
            mQ = pack_gqa_layout(mQ, self.qhead_per_kvhead, nheads_kv, head_idx=2)
            mO = pack_gqa_layout(mO, self.qhead_per_kvhead, nheads_kv, head_idx=2)
            if const_expr(mLSE is not None):
                mLSE = pack_gqa_layout(mLSE, self.qhead_per_kvhead, nheads_kv, head_idx=1)
        # TileScheduler for varlen, simple grid for non-varlen
        if const_expr(mCuSeqlensQ is not None or mSeqUsedQ is not None):
            TileScheduler = SingleTileVarlenScheduler
        else:
            TileScheduler = SingleTileScheduler
        num_batch = (
            mCuSeqlensQ.shape[0] - 1
            if const_expr(mCuSeqlensQ is not None)
            else cute.size(mQ.shape[3])
        )
        tile_sched_args = TileSchedulerArguments(
            num_block=cute.ceil_div(cute.size(mQ.shape[0]), self.tile_m),
            num_head=cute.size(mQ.shape[2]),
            num_batch=num_batch,
            num_splits=1,
            seqlen_k=0,
            headdim=mQ.shape[1],
            headdim_v=mV.shape[1],
            total_q=cute.size(mQ.shape[0])
            if const_expr(mCuSeqlensQ is not None)
            else cute.size(mQ.shape[0]) * cute.size(mQ.shape[3]),
            tile_shape_mn=(self.tile_m, self.tile_n),
            qhead_per_kvhead_packgqa=self.qhead_per_kvhead if const_expr(self.pack_gqa) else 1,
            mCuSeqlensQ=mCuSeqlensQ,
            mSeqUsedQ=mSeqUsedQ,
            cu_total_m_blocks_ptr=mCuTotalMBlocks,
            cu_total_splits_m_blocks_ptr=mCuTotalSplitsMBlocks,
        )
        tile_sched_params = TileScheduler.to_underlying_arguments(tile_sched_args)
        grid_dim = TileScheduler.get_grid_shape(tile_sched_params)
        softmax_scale_log2, softmax_scale = utils.compute_softmax_scale_log2(softmax_scale, self.score_mod)
        fastdiv_mods = utils.compute_fastdiv_mods(mQ, mK, self.qhead_per_kvhead, self.pack_gqa, aux_data.tensors)

        self.kernel(
            mQ,
            mK,
            mV,
            mO,
            mLSE,
            mCuSeqlensQ,
            mCuSeqlensK,
            mSeqUsedQ,
            mSeqUsedK,
            softmax_scale_log2,
            softmax_scale,
            window_size_left,
            window_size_right,
            self.sQ_layout,
            self.sK_layout,
            self.sV_layout,
            self.sO_layout,
            self.sP_layout,
            self.gmem_tiled_copy_Q,
            self.gmem_tiled_copy_K,
            self.gmem_tiled_copy_V,
            self.gmem_tiled_copy_O,
            tiled_mma_qk,
            tiled_mma_pv,
            SharedStorage,
            tile_sched_params,
            TileScheduler,
            aux_data,
            fastdiv_mods,
        ).launch(
            grid=grid_dim,
            block=[self.num_threads, 1, 1],
            smem=SharedStorage.size_in_bytes(),
            stream=stream,
        )

    @cute.kernel
    def kernel(
        self,
        mQ: cute.Tensor,
        mK: cute.Tensor,
        mV: cute.Tensor,
        mO: cute.Tensor,
        mLSE: Optional[cute.Tensor],
        mCuSeqlensQ: Optional[cute.Tensor],
        mCuSeqlensK: Optional[cute.Tensor],
        mSeqUsedQ: Optional[cute.Tensor],
        mSeqUsedK: Optional[cute.Tensor],
        softmax_scale_log2: Float32,
        softmax_scale: Optional[Float32],
        window_size_left: Optional[Int32],
        window_size_right: Optional[Int32],
        sQ_layout: cute.ComposedLayout,
        sK_layout: cute.ComposedLayout,
        sV_layout: cute.ComposedLayout,
        sO_layout: cute.ComposedLayout,
        sP_layout: cute.ComposedLayout | None,
        gmem_tiled_copy_Q: cute.TiledCopy,
        gmem_tiled_copy_K: cute.TiledCopy,
        gmem_tiled_copy_V: cute.TiledCopy,
        gmem_tiled_copy_O: cute.TiledCopy,
        tiled_mma_qk: cute.TiledMma,
        tiled_mma_pv: cute.TiledMma,
        SharedStorage: cutlass.Constexpr,
        tile_sched_params,
        TileScheduler: cutlass.Constexpr[Callable],
        aux_data: AuxData = AuxData(),
        fastdiv_mods=None,
    ):
        # Thread index, block index
        tidx, _, _ = cute.arch.thread_idx()

        tile_scheduler = TileScheduler.create(tile_sched_params)
        work_tile = tile_scheduler.initial_work_tile_info()
        m_block, num_head, batch_size, _ = work_tile.tile_idx

        block_info = BlockInfo(
            self.tile_m,
            self.tile_n,
            self.is_causal,
            self.is_local,
            False,  # is_split_kv
            window_size_left,
            window_size_right,
            qhead_per_kvhead_packgqa=self.qhead_per_kvhead if const_expr(self.pack_gqa) else 1,
        )
        seqlen = SeqlenInfoQK.create(
            batch_idx=batch_size,
            seqlen_q_static=mQ.shape[0] if const_expr(not self.pack_gqa) else mQ.shape[0][1],
            seqlen_k_static=mK.shape[0],
            mCuSeqlensQ=mCuSeqlensQ,
            mCuSeqlensK=mCuSeqlensK,
            mSeqUsedQ=mSeqUsedQ,
            mSeqUsedK=mSeqUsedK,
        )
        n_block_min, n_block_max = block_info.get_n_block_min_max(seqlen, m_block)
        # For varlen, wasted grid tiles (where batch_idx >= num_batch) will have
        # seqlen_q=seqlen_k=0 and n_block_max=0.  Clamp to 0 so we don't use a
        # negative block index for K/V loads; the load/store predicates already
        # guard all memory accesses when seqlen is 0.
        n_block = cutlass.max(n_block_max - 1, 0)

        # ///////////////////////////////////////////////////////////////////////////////
        # Get the appropriate tiles for this thread block.
        # ///////////////////////////////////////////////////////////////////////////////
        blkQ_shape = (self.tile_m, self.tile_hdim)
        blkK_shape = (self.tile_n, self.tile_hdim)
        blkV_shape = (self.tile_n, self.tile_hdimv)
        num_head_kv = num_head if const_expr(self.pack_gqa) else num_head // self.qhead_per_kvhead
        mQ_cur = seqlen.offset_batch_Q(mQ, batch_size, dim=3)[None, None, num_head]
        if const_expr(not seqlen.has_cu_seqlens_k):
            mK_cur = mK[None, None, num_head_kv, batch_size]
            mV_cur = mV[None, None, num_head_kv, batch_size]
        else:
            mK_cur = cute.domain_offset((seqlen.offset_k, 0), mK[None, None, num_head_kv])
            mV_cur = cute.domain_offset((seqlen.offset_k, 0), mV[None, None, num_head_kv])
        if const_expr(not self.pack_gqa):
            gQ = cute.local_tile(mQ_cur, blkQ_shape, (m_block, 0))
        gK = cute.local_tile(mK_cur, blkK_shape, (None, 0))
        gV = cute.local_tile(mV_cur, blkV_shape, (None, 0))

        # ///////////////////////////////////////////////////////////////////////////////
        # Get shared memory buffer
        # ///////////////////////////////////////////////////////////////////////////////
        smem = cutlass.utils.SmemAllocator()
        storage = smem.allocate(SharedStorage)
        sQ = storage.sQ.get_tensor(sQ_layout)
        sK = storage.sK.get_tensor(sK_layout)
        if const_expr(not self.Q_in_regs):
            sV = storage.sV.get_tensor(sV_layout)
        else:
            sV = cute.make_tensor(cute.recast_ptr(sQ.iterator, dtype=self.dtype), sV_layout)
        # Transpose view of V to tensor with layout (head_dim_v, tile_n) for tiled mma
        sVt = layout_utils.transpose_view(sV)

        gmem_thr_copy_K = gmem_tiled_copy_K.get_slice(tidx)
        gmem_thr_copy_V = gmem_tiled_copy_V.get_slice(tidx)
        # (CPY_Atom, CPY_N, CPY_K, n_block)
        tKsK, tKgK = gmem_thr_copy_K.partition_D(sK), gmem_thr_copy_K.partition_S(gK)
        # (CPY_Atom, CPY_N, CPY_K, n_block)
        tVsV, tVgV = gmem_thr_copy_V.partition_D(sV), gmem_thr_copy_V.partition_S(gV)

        # ///////////////////////////////////////////////////////////////////////////////
        # Tile MMA compute thread partitions and allocate accumulators
        # ///////////////////////////////////////////////////////////////////////////////
        thr_mma_qk = tiled_mma_qk.get_slice(tidx)
        thr_mma_pv = tiled_mma_pv.get_slice(tidx)
        tSrQ = thr_mma_qk.make_fragment_A(thr_mma_qk.partition_A(sQ))
        tSrK = thr_mma_qk.make_fragment_B(thr_mma_qk.partition_B(sK[None, None, 0]))
        tOrVt = thr_mma_pv.make_fragment_B(thr_mma_pv.partition_B(sVt[None, None, 0]))
        acc_shape_O = thr_mma_pv.partition_shape_C((self.tile_m, self.tile_hdimv))
        acc_O = cute.make_rmem_tensor(acc_shape_O, Float32)
        acc_O.fill(0.0)

        # ///////////////////////////////////////////////////////////////////////////////
        # Smem copy atom tiling
        # ///////////////////////////////////////////////////////////////////////////////
        smem_copy_atom_QK = cute.make_copy_atom(
            warp.LdMatrix8x8x16bOp(transpose=False, num_matrices=4),
            self.dtype,
        )
        smem_copy_atom_V = cute.make_copy_atom(
            warp.LdMatrix8x8x16bOp(transpose=True, num_matrices=4),
            self.dtype,
        )
        smem_thr_copy_Q = utils.make_tiled_copy_A(smem_copy_atom_QK, tiled_mma_qk).get_slice(tidx)
        smem_thr_copy_K = utils.make_tiled_copy_B(smem_copy_atom_QK, tiled_mma_qk).get_slice(tidx)
        smem_thr_copy_V = utils.make_tiled_copy_B(smem_copy_atom_V, tiled_mma_pv).get_slice(tidx)

        tSsQ = smem_thr_copy_Q.partition_S(sQ)
        tSsK = smem_thr_copy_K.partition_S(sK)
        tOsVt = smem_thr_copy_V.partition_S(sVt)

        # ///////////////////////////////////////////////////////////////////////////////
        # Predicate: Mark indices that need to copy when problem_shape isn't a multiple
        # of tile_shape
        # ///////////////////////////////////////////////////////////////////////////////
        # Construct identity layout for KV
        cK = cute.make_identity_tensor((self.tile_n, self.tile_hdim))
        tKcK = gmem_thr_copy_K.partition_S(cK)
        t0KcK = gmem_thr_copy_K.get_slice(0).partition_S(cK)
        if const_expr(self.tile_hdim == self.tile_hdimv):
            tVcV = tKcK
            t0VcV = t0KcK
        else:
            cV = cute.make_identity_tensor((self.tile_n, self.tile_hdimv))
            tVcV = gmem_thr_copy_V.partition_S(cV)
            t0VcV = gmem_thr_copy_V.get_slice(0).partition_S(cV)
        # Allocate predicate tensors for m and n, here we only allocate the tile of k, and
        # use "if" on the mn dimension.
        # This is to reduce register pressure and gets 2-3% performance gain.
        tKpK = utils.predicate_k(tKcK, limit=mK.shape[1])
        if const_expr(self.same_hdim_kv):
            tVpV = tKpK
        else:
            tVpV = utils.predicate_k(tVcV, limit=mV.shape[1])

        # shape: (atom_v_m * rest_m)
        softmax = Softmax.create(
            softmax_scale_log2,
            num_rows=acc_O.shape[0][0] * acc_O.shape[1],
            softmax_scale=softmax_scale,
        )
        softmax.reset()

        # group parameters for compute_one_n_block
        mma_params = SimpleNamespace(
            thr_mma_qk=thr_mma_qk,
            thr_mma_pv=thr_mma_pv,
            tSrQ=tSrQ,
            tSrK=tSrK,
            tOrVt=tOrVt,
            acc_O=acc_O,
        )
        smem_copy_params = SimpleNamespace(
            smem_thr_copy_Q=smem_thr_copy_Q,
            smem_thr_copy_K=smem_thr_copy_K,
            smem_thr_copy_V=smem_thr_copy_V,
            tSsQ=tSsQ,
            tSsK=tSsK,
            tOsVt=tOsVt,
        )
        load_K = partial(
            self.load_K, gmem_tiled_copy_K, tKgK, tKsK, tKcK, t0KcK, tKpK, seqlen=seqlen.seqlen_k
        )
        load_V = partial(
            self.load_V, gmem_tiled_copy_V, tVgV, tVsV, tVcV, t0VcV, tVpV, seqlen=seqlen.seqlen_k
        )

        compute_one_n_block = partial(
            self.compute_one_n_block,
            mma_params=mma_params,
            smem_copy_params=smem_copy_params,
            softmax=softmax,
            load_K=load_K,
            load_V=load_V,
            score_mod=self.score_mod,
            batch_idx=batch_size,
            head_idx=num_head,
            m_block=m_block,
            aux_data=aux_data,
            fastdiv_mods=fastdiv_mods,
        )

        # ///////////////////////////////////////////////////////////////////////////////
        # Prologue
        # ///////////////////////////////////////////////////////////////////////////////
        # Start async loads of the last mn-tile, where we take care of the mn residue
        gmem_thr_copy_Q = gmem_tiled_copy_Q.get_slice(tidx)
        if const_expr(not self.pack_gqa):
            self.load_Q(gmem_thr_copy_Q, gQ, sQ, m_block, seqlen=seqlen.seqlen_q, headdim=mQ.shape[1])
        else:
            pack_gqa = PackGQA(self.tile_m, self.tile_hdim, self.check_hdim_oob, self.qhead_per_kvhead)
            pack_gqa.load_Q(mQ_cur, sQ, gmem_tiled_copy_Q, tidx, m_block, seqlen.seqlen_q)
        cute.arch.cp_async_commit_group()

        def preprocess_Q():
            cute.arch.cp_async_wait_group(self.num_stages * 2 - 1)
            if const_expr(self.Q_in_regs):
                cute.arch.barrier()
                tSrQ_copy_view = smem_thr_copy_Q.retile(tSrQ)
                cute.copy(smem_thr_copy_Q, tSsQ, tSrQ_copy_view)

        # If Q_in_regs, we load Q, then load 1 stage of K, then (optionally) rotate Q and
        # read from smem_q to registers, then load V.
        # If !Q_in_regs, we load Q, load all stages of K & V, then (optionally) rotate Q.
        if const_expr(self.Q_in_regs):
            load_K(n_block, smem_pipe_write=0, need_predicates=True)
            cute.arch.cp_async_commit_group()
            preprocess_Q()
            cute.arch.barrier()  # Make sure all threads have read smem_q before loading V

        for stage in cutlass.range_constexpr(self.num_stages):
            if const_expr(not self.Q_in_regs or stage > 0):
                if stage == 0 or n_block - stage >= 0:
                    load_K(n_block - stage, smem_pipe_write=stage, need_predicates=stage == 0)
                cute.arch.cp_async_commit_group()
            if const_expr(stage < self.num_stages - 1):
                if stage == 0 or n_block - stage >= 0:
                    load_V(n_block - stage, smem_pipe_write=stage, need_predicates=stage == 0)
                cute.arch.cp_async_commit_group()
        if const_expr(not self.Q_in_regs):
            preprocess_Q()

        # ///////////////////////////////////////////////////////////////////////////////
        # Mainloop
        # ///////////////////////////////////////////////////////////////////////////////
        # Start processing of the first n-block.
        # For performance reason, we separate out two kinds of iterations:
        # those that need masking on S, and those that don't.
        # We need masking on S for the very last block when K and V has length not multiple of tile_n.
        # We also need masking on S if it's causal, for the last several blocks.
        mask = AttentionMask(
            self.tile_m,
            self.tile_n,
            seqlen,
            window_size_left,
            window_size_right,
            self.qhead_per_kvhead if const_expr(self.pack_gqa) else 1,
        )
        mask_fn = partial(
            mask.apply_mask,
            batch_idx=batch_size,
            head_idx=num_head,
            m_block=m_block,
            thr_mma=thr_mma_qk,
            mask_causal=self.is_causal,
            mask_local=self.is_local,
            aux_data=aux_data,
            fastdiv_mods=fastdiv_mods if const_expr(self.mask_mod is not None) else None,
        )

        # First iteration with seqlen masking
        smem_pipe_read = Int32(0)
        smem_pipe_write = Int32(self.num_stages - 1)
        compute_one_n_block(
            n_block,
            smem_pipe_read,
            smem_pipe_write,
            is_first_n_block=True,
            seqlen=seqlen,
            mask_fn=partial(mask_fn, mask_mod=self.mask_mod, mask_seqlen=True),
        )
        smem_pipe_read = self.advance_pipeline(smem_pipe_read)
        smem_pipe_write = self.advance_pipeline(smem_pipe_write)
        # Next couple of iterations with causal masking
        if const_expr(self.is_causal or self.is_local):
            n_block_min_causal_local_mask = block_info.get_n_block_min_causal_local_mask(
                seqlen, m_block, n_block_min
            )
            for n_tile in cutlass.range(n_block_max - 1 - n_block_min_causal_local_mask, unroll=1):
                n_block = n_block_max - 2 - n_tile
                compute_one_n_block(
                    n_block,
                    smem_pipe_read,
                    smem_pipe_write,
                    seqlen=seqlen,
                    mask_fn=partial(mask_fn, mask_mod=self.mask_mod, mask_seqlen=True),
                )
                smem_pipe_read = self.advance_pipeline(smem_pipe_read)
                smem_pipe_write = self.advance_pipeline(smem_pipe_write)
        # The remaining iterations have no masking
        for n_tile in cutlass.range(n_block, unroll=1):
            compute_one_n_block(
                n_block - n_tile - 1, smem_pipe_read, smem_pipe_write,
                seqlen=seqlen, is_first_n_block=False,
                mask_fn=partial(mask_fn, mask_mod=self.mask_mod, mask_seqlen=False)
            )
            smem_pipe_read = self.advance_pipeline(smem_pipe_read)
            smem_pipe_write = self.advance_pipeline(smem_pipe_write)
        # TODO: local

        # normalize acc_O by row_sum and calculate the lse
        row_scale = softmax.finalize()
        softmax.rescale_O(acc_O, row_scale)

        # ///////////////////////////////////////////////////////////////////////////////
        # Epilogue
        # ///////////////////////////////////////////////////////////////////////////////
        # reuse sQ's data iterator
        sO = cute.make_tensor(sQ.iterator, sO_layout)
        self.epilogue(
            acc_O,
            softmax.row_sum,
            mO,
            mLSE,
            sO,
            seqlen,
            gmem_tiled_copy_O,
            None,
            tiled_mma_pv,
            tidx,
            m_block,
            num_head,
            batch_size,
        )

    @cute.jit
    def compute_one_n_block(
        self,
        n_block: Int32,
        smem_pipe_read: Int32,
        smem_pipe_write: Int32,
        mma_params: SimpleNamespace,
        smem_copy_params: SimpleNamespace,
        softmax: Softmax,
        load_K: Callable,
        load_V: Callable,
        score_mod: Callable | None,
        batch_idx: cutlass.Int32,
        head_idx: cutlass.Int32,
        m_block: cutlass.Int32,
        seqlen: SeqlenInfoQK,
        aux_data: AuxData = AuxData(),
        fastdiv_mods=None,
        mask_fn: Optional[Callable] = None,
        is_first_n_block: cutlass.Constexpr = False,
        check_inf: cutlass.Constexpr = True,
    ):
        """Compute one n_block of S/O.

        This function provides different variants for processing the first n block versus
        subsequent blocks.
        """

        def sync():
            cute.arch.cp_async_wait_group(self.num_stages * 2 - 2)
            cute.arch.barrier()

        acc_shape_S = mma_params.thr_mma_qk.partition_shape_C((self.tile_m, self.tile_n))
        acc_S = cute.make_rmem_tensor(acc_shape_S, Float32)
        acc_S.fill(0.0)
        # wait for smem tile QK before mma calculation for S
        sync()

        # need predicates for the first tile
        def load_V_next():
            if self.num_stages == 1 or n_block - self.num_stages + 1 >= 0:
                load_V(
                    n_block - self.num_stages + 1,
                    smem_pipe_write,
                    need_predicates=is_first_n_block and self.num_stages == 1,
                )
            cute.arch.cp_async_commit_group()

        load_V_next()
        sm80_utils.gemm(
            mma_params.thr_mma_qk,
            acc_S,
            mma_params.tSrQ,
            mma_params.tSrK,
            smem_copy_params.tSsQ,
            smem_copy_params.tSsK[
                None, None, None, smem_pipe_read if const_expr(self.num_stages > 1) else 0
            ],
            smem_copy_params.smem_thr_copy_Q,
            smem_copy_params.smem_thr_copy_K,
            # hook_fn=load_V_next,
            A_in_regs=self.Q_in_regs,
        )
        if const_expr(score_mod is not None):
            self.apply_score_mod(
                mma_params.thr_mma_qk,
                batch_idx,
                head_idx,
                m_block,
                acc_S,
                n_block,
                softmax_scale=softmax.softmax_scale,
                seqlen=seqlen,
                aux_data=aux_data,
                fastdiv_mods=fastdiv_mods,
            )

        smem_pipe_write = self.advance_pipeline(smem_pipe_write)

        def load_K_next():
            if n_block - self.num_stages >= 0:
                load_K(n_block - self.num_stages, smem_pipe_write, need_predicates=False)
            cute.arch.cp_async_commit_group()

        # wait for smem tile V for O
        if const_expr(self.num_stages == 1):
            sync()
            load_K_next()
        if const_expr(mask_fn is not None):
            mask_fn(acc_S, n_block=n_block)
        row_scale = softmax.online_softmax(acc_S, is_first=is_first_n_block, check_inf=check_inf)
        softmax.rescale_O(mma_params.acc_O, row_scale)
        rP = cute.make_fragment_like(acc_S, self.dtype)
        rP.store(acc_S.load().to(self.dtype))
        tOrP = layout_utils.reshape_acc_to_frgA(rP)
        if const_expr(self.num_stages > 1):
            sync()
            load_K_next()
        sm80_utils.gemm_rs(
            mma_params.thr_mma_pv,
            mma_params.acc_O,
            tOrP,
            mma_params.tOrVt,
            smem_copy_params.tOsVt[
                None, None, None, smem_pipe_read if const_expr(self.num_stages > 1) else 0
            ],
            smem_copy_params.smem_thr_copy_V,
            # hook_fn=load_K_next,
        )
        # if const_expr(self.num_stages > 1):
        #     load_K_next()
    @cute.jit
    def apply_score_mod(
        self,
        thr_mma_qk,
        batch_idx,
        head_idx,
        m_block,
        acc_S,
        n_block,
        softmax_scale,
        seqlen,
        aux_data: AuxData = AuxData(),
        fastdiv_mods=None,
    ):
        # Prepare index tensor
        cS = cute.make_identity_tensor((self.tile_m, self.tile_n))
        cS = cute.domain_offset((m_block * self.tile_m, n_block * self.tile_n), cS)
        tScS = thr_mma_qk.partition_C(cS)

        apply_score_mod_inner(
            acc_S,
            tScS,
            self.score_mod,
            batch_idx,
            head_idx,
            softmax_scale,
            self.score_vec_size,
            self.qk_acc_dtype,
            aux_data,
            fastdiv_mods,
            seqlen_info=seqlen,
            constant_q_idx=None,
            qhead_per_kvhead=self.qhead_per_kvhead if const_expr(self.pack_gqa) else 1,
        )


# SM90 forward pass moved to flash_fwd_sm90.py; re-export for backward compatibility
def __getattr__(name):
    if name == "FlashAttentionForwardSm90":
        from flash_attn.cute.flash_fwd_sm90 import FlashAttentionForwardSm90
        return FlashAttentionForwardSm90
    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")