Bus: Enable mmap fastmem on uppermost KSEG0 mirror

Instead of mapping all the RAM mirrors, we only map the KSEG0
uppermost mirror. This is where some games place their stack, so
we avoid the backpatching overhead/slowdown, but don't pay the
cost of 4x the mprotect() calls when a page's protection changes,
which can have a non-trivial impact on slow ARM devices.

src/core/bus.cpp

@@ -560,7 +560,6 @@ void Bus::MapFastmemViews()
   {
 #ifdef ENABLE_MMAP_FASTMEM
     auto MapRAM = [](u32 base_address) {
-      // No need to check mapped RAM range here, we only ever fastmem map the first 2MB.
       u8* map_address = s_fastmem_arena.BasePointer() + base_address;
       if (!s_fastmem_arena.Map(s_shmem_handle, 0, map_address, g_ram_size, PageProtect::ReadWrite)) [[unlikely]]
       {
@@ -570,7 +569,8 @@ void Bus::MapFastmemViews()
       }
 
       // mark all pages with code as non-writable
-      for (u32 i = 0; i < static_cast<u32>(g_ram_code_bits.size()); i++)
+      const u32 page_count = g_ram_size >> HOST_PAGE_SHIFT;
+      for (u32 i = 0; i < page_count; i++)
       {
         if (g_ram_code_bits[i])
         {
@@ -595,6 +595,16 @@ void Bus::MapFastmemViews()
 
     // KSEG1 - uncached
     MapRAM(0xA0000000);
+
+    // Mirrors of 2MB
+    if (g_ram_size == RAM_2MB_SIZE)
+    {
+      // Instead of mapping all the RAM mirrors, we only map the KSEG0 uppermost mirror.
+      // This is where some games place their stack, so we avoid the backpatching overhead/slowdown,
+      // but don't pay the cost of 4x the mprotect() calls when a page's protection changes, which
+      // can have a non-trivial impact on slow ARM devices.
+      MapRAM(0x80600000);
+    }
 #else
     Panic("MMap fastmem should not be selected on this platform.");
 #endif
@@ -675,7 +685,8 @@ bool Bus::CanUseFastmemForAddress(VirtualMemoryAddress address)
     {
       // Currently since we don't map the mirrors, don't use fastmem for them.
       // This is because the swapping of page code bits for SMC is too expensive.
-      return (paddr < g_ram_size);
+      // Except for the uppermost mirror in KSEG0, see above.
+      return (paddr < g_ram_size) || (address >= 0x80600000 && address < 0x80800000);
     }
 #endif