Adopt 2-entry direct-mapped page cache

Replace the previous 1-entry direct-mapped design with a 2-entry
direct-mapped cache using hash-based indexing (same parity hash as
cache_load). This allows two hot virtual pages to coexist without
thrashing.

Measurement shows that the number of virtual-to-physical translations
during instruction fetch (mmu_translate() calls) decreased by ~10%.

Author: yy214123

riscv.c

@@ -190,7 +190,8 @@ static inline void ic_invalidate_all(hart_t *vm)
 
 void mmu_invalidate(hart_t *vm)
 {
-    vm->cache_fetch.n_pages = 0xFFFFFFFF;
+    vm->cache_fetch[0].n_pages = 0xFFFFFFFF;
+    vm->cache_fetch[1].n_pages = 0xFFFFFFFF;
     /* Invalidate all 8 sets × 2 ways for load cache */
     for (int set = 0; set < 8; set++) {
         for (int way = 0; way < 2; way++)
@@ -388,7 +389,8 @@ static void mmu_fetch(hart_t *vm, uint32_t addr, uint32_t *value)
 
     /* cache miss, Continue using the original va->pa*/
     uint32_t vpn = addr >> RV_PAGE_SHIFT;
-    if (unlikely(vpn != vm->cache_fetch.n_pages)) {
+    uint32_t index = __builtin_parity(vpn) & 0x1;
+    if (unlikely(vpn != vm->cache_fetch[index].n_pages)) {
         mmu_translate(vm, &addr, (1 << 3), (1 << 6), false, RV_EXC_FETCH_FAULT,
                       RV_EXC_FETCH_PFAULT);
         if (vm->error)
@@ -397,15 +399,16 @@ static void mmu_fetch(hart_t *vm, uint32_t addr, uint32_t *value)
         vm->mem_fetch(vm, addr >> RV_PAGE_SHIFT, &page_addr);
         if (vm->error)
             return;
-        vm->cache_fetch.n_pages = vpn;
-        vm->cache_fetch.page_addr = page_addr;
+        vm->cache_fetch[index].n_pages = vpn;
+        vm->cache_fetch[index].page_addr = page_addr;
     }
 
-    *value = vm->cache_fetch.page_addr[(addr >> 2) & MASK(RV_PAGE_SHIFT - 2)];
+    *value =
+        vm->cache_fetch[index].page_addr[(addr >> 2) & MASK(RV_PAGE_SHIFT - 2)];
 
     /* fill into the cache */
     uint32_t block_off = (addr & RV_PAGE_MASK) & ~IC_BLOCK_MASK;
-    blk->base = (const uint8_t *) vm->cache_fetch.page_addr + block_off;
+    blk->base = (const uint8_t *) vm->cache_fetch[index].page_addr + block_off;
     blk->tag = tag;
     blk->valid = true;
 }

riscv.h

@@ -140,7 +140,8 @@ struct __hart_internal {
      */
     uint32_t exc_cause, exc_val;
 
-    mmu_fetch_cache_t cache_fetch;
+    /* 2-entry direct-mapped with hash-based indexing */
+    mmu_fetch_cache_t cache_fetch[2];
     /* 8-set × 2-way set-associative cache with 3-bit parity hash indexing */
     mmu_cache_set_t cache_load[8];
     /* 8-set × 2-way set-associative cache for store operations */