Fix system emulation reboot

Previously, the SBI RST extension did not distinguish between reboot and
shutdown type. When a userspace reboot command was issued, the hart was
incorrectly halted as if it were a shutdown.

These changes fix the issue by properly detecting and handling two
reboot types: cold and warm. Each type has its own handler:
rv_cold_reboot() and rv_warm_reboot(). Since the initial system
power-on is treated as a cold reboot, the initialization code has been
refactored to share logic with the reboot path, adhering to the DRY
principle. Additionally, device reset helper functions (plic_reset(),
u8250_reset()) are introduced to support peripheral reinitialization
during reboot.

Key changes:
1. Rename rv_reset() to rv_cold_reboot() - Full system reset including
processor, memory, and all peripherals. The initial power-on is treated
as a cold reboot.

2. Introduce rv_warm_reboot() - Faster reboot that only resets processor
and memory, skipping peripheral reinitialization. Can be triggered via
echo "warm" > /sys/kernel/reboot/mode in guestOS.

3. Refactor boot image loading - Extract load_boot_images() helper to
load kernel, DTB, and initrd, reducing code duplication between cold and
warm reboot paths.

4. Introduce rv_reset_hart() - Static helper function to reset only hart
state (GPRs, CSRs, PC), shared by both reboot modes.

5. Add reboot-safe resource management - Add "check for reboot" comments
throughout initialization to reuse already-allocated resources (memory,
fd_map, PLIC, UART, vblk, block_map) instead of re-allocating.

6. Use calloc for vblk/disk arrays - Changed from malloc to calloc so
pointers are initialized to NULL, simplifying reboot checks.

7. Use setjmp/longjmp for clean reboot - Reboot rewrites all registers,
causing call stack values to become stale. setjmp in rv_step()
establishes a return point, longjmp after reboot provides a clean call
stack.

8. Add plic_reset() and u8250_reset() - New device reset functions to
reinitialize state without free/realloc.

9. Add sbi_rst_type_str() and sbi_rst_reason_str() - Helper functions
for human-readable SBI reset type/reason in dmesg.

Author: ChinYikMing

src/devices/plic.c

@@ -89,3 +89,8 @@ void plic_delete(plic_t *plic)
 {
     free(plic);
 }
+
+void plic_reset(plic_t *plic)
+{
+    memset(plic, 0, sizeof(plic_t));
+}

src/devices/plic.h

@@ -39,3 +39,6 @@ plic_t *plic_new();
 
 /* delete a PLIC instance */
 void plic_delete(plic_t *plic);
+
+/* reset a PLIC instance */
+void plic_reset(plic_t *plic);

src/devices/uart.c

@@ -197,3 +197,8 @@ void u8250_delete(u8250_state_t *uart)
 {
     free(uart);
 }
+
+void u8250_reset(u8250_state_t *uart)
+{
+    memset(uart, 0, sizeof(u8250_state_t));
+}

src/devices/uart.h

@@ -49,3 +49,6 @@ u8250_state_t *u8250_new();
 
 /* delete a UART instance */
 void u8250_delete(u8250_state_t *uart);
+
+/* reset a UART instance */
+void u8250_reset(u8250_state_t *uart);

src/devices/virtio-blk.c

@@ -403,8 +403,12 @@ uint32_t *virtio_blk_init(virtio_blk_state_t *vblk,
     vblk->disk_fd = -1;
 
     /* Allocate memory for the private member */
-    vblk->priv = calloc(1, sizeof(struct virtio_blk_config));
-    assert(vblk->priv);
+    if (vblk->priv) { /* check for reboot */
+        memset(vblk->priv, 0, sizeof(struct virtio_blk_config));
+    } else {
+        vblk->priv = calloc(1, sizeof(struct virtio_blk_config));
+        assert(vblk->priv);
+    }
 
     /* No disk image is provided */
     if (!disk_file) {

src/emulate.c

@@ -1098,7 +1098,6 @@ static void rv_check_interrupt(riscv_t *rv)
     vm_attr_t *attr = PRIV(rv);
     if (peripheral_update_ctr-- == 0) {
         peripheral_update_ctr = 64;
-
 #if defined(__EMSCRIPTEN__)
     escape_seq:
 #endif
@@ -1148,7 +1147,24 @@ void rv_step(void *arg)
     /* find or translate a block for starting PC */
     const uint64_t cycles_target = rv->csr_cycle + cycles;
 
-    /* loop until hitting the cycle target */
+#if RV32_HAS(SYSTEM) && !RV32_HAS(ELF_LOADER)
+    /* Set up the jump point for handling reboots */
+    if (setjmp(rv->reboot_jmp) != 0) {
+        /* longjmp to here after a reboot happens */
+#if !RV32_HAS(JIT)
+        need_clear_block_map = false;
+#endif
+        is_branch_taken = false;
+        reloc_enable_mmu_jalr_addr = 0;
+        reloc_enable_mmu = false;
+        need_retranslate = false;
+        need_handle_signal = false;
+        prev = NULL;
+        last_pc = 0;
+    }
+#endif
+
+    /* loop until hitting the cycle target or hart is halted */
     while (rv->csr_cycle < cycles_target && !rv->halt) {
 #if RV32_HAS(SYSTEM) && !RV32_HAS(ELF_LOADER)
         /* check for any interrupt after every block emulation */