[dv] Document llvm-snippy flow

Document the llvm-snippy flow from the core_ibex user-facing README,
including the required tools, Make entry points, test selection knobs,
and coverage targets.

Author: AnastasiyaChernikova

dv/uvm/core_ibex/README.md

@@ -27,9 +27,118 @@ export RISCV_GCC=riscv32-unknown-elf-gcc
 export RISCV_OBJCOPY=riscv32-unknown-elf-objcopy
 ```
 
+For the optional llvm-snippy flow, set `SC_SNIPPY_PATH` to the llvm-snippy
+installation directory:
+
+```bash
+export SC_SNIPPY_PATH=/path/to/llvm-snippy
+```
+
+The Snippy flow expects the executable at `$SC_SNIPPY_PATH/llvm-snippy`.
+More Snippy-specific setup notes are documented in `snippy_cmake/README.md`.
+
 ## Running tests
 
 To run tests you can make variations of the following command, where you replace `$TEST_NAME` with the test (or a series of comma-separated tests) that you would like to run as specified in `dv/uvm/core_ibex/riscv_dv_extension/testlist.yaml`:
 ```bash
 make --keep-going IBEX_CONFIG=opentitan SIMULATOR=xlm ISS=spike ITERATIONS=1 SEED=1 TEST=$TEST_NAME WAVES=0 COV=0
+```
+
+## Running Snippy tests
+
+The custom Snippy flow can be launched through the Ibex Makefile with
+`run_snippy`.
+
+Example:
+
+```bash
+make run_snippy \
+  OUT=out \
+  COV=1 \
+  SIMULATOR=vcs \
+  IBEX_CONFIG=opentitan \
+  RV32ZC=ibex_pkg::RV32Zca \
+  SNIPPY_ITERATIONS=3 \
+  SNIPPY_TEST="layout_arith, jalr, rem_div" \
+  SNIPPY_YAML_DIR=snippy/yaml_tests
+```
+
+`SNIPPY_TEST` selects YAML files from `SNIPPY_YAML_DIR`.
+
+`SNIPPY_TEST` is a comma-separated list of Snippy YAML layouts. The accepted
+forms include `layout_arith`, `arith`, and `layout_arith.yaml`.
+
+`SNIPPY_ITERATIONS` controls the number of Snippy runs for each selected layout.
+
+Snippy runtime results are written into the standard Ibex output tree:
+
+```text
+out/run/tests/snippy_<layout>.<seed>/
+```
+
+The CMake-side Snippy intermediate files and summary are written under:
+
+```text
+out/snippy_cmake_build/
+```
+
+## Collecting combined riscv-dv and Snippy coverage
+
+To collect one combined coverage report from both riscv-dv and Snippy runs, use
+the same `OUT` directory and enable `COV=1` for both flows.
+
+First, run riscv-dv only up to RTL simulation:
+
+```bash
+make run \
+  GOAL=rtl_sim_run \
+  OUT=out \
+  COV=1 \
+  SIMULATOR=vcs \
+  IBEX_CONFIG=opentitan \
+  RV32ZC=ibex_pkg::RV32Zca \
+  TEST=riscv_arithmetic_basic_test
+```
+
+Using `GOAL=rtl_sim_run` avoids collecting an intermediate coverage report
+before the Snippy tests have been added.
+
+Then run Snippy into the same `OUT` directory:
+
+```bash
+make run_snippy \
+  OUT=out \
+  COV=1 \
+  SIMULATOR=vcs \
+  IBEX_CONFIG=opentitan \
+  RV32ZC=ibex_pkg::RV32Zca \
+  SNIPPY_ITERATIONS=3 \
+  SNIPPY_TEST="layout_arith, jalr, rem_div"
+```
+
+Finally, merge coverage once:
+
+```bash
+make coverage_snippy OUT=out
+```
+
+The final report is written to:
+
+```text
+out/run/coverage/report/
+```
+
+## Cleaning generated files
+
+To remove generated outputs:
+
+```bash
+make clean OUT=out
+```
+
+This also removes the Snippy CMake build directory when it is located under
+`OUT`, for example:
+
+```text
+out/snippy_cmake_build/
 ```