[video_core] nvn descriptor layout fix (#3206)

Yxzx presumes this:
// The NVN driver buffer (index 0) is known to pack the SSBO address followed by its size.
But in MCI i`ve discovered that there are no sizes, both registers are GPU addresses (hence the 2.8gb allocation, it was an address actually)

Method could be much simpler but for safety i`ve routed both old and new worlds.

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/3206
Reviewed-by: Caio Oliveira <caiooliveirafarias0@gmail.com>
Reviewed-by: Lizzie <lizzie@eden-emu.dev>
Co-authored-by: xbzk <xbzk@eden-emu.dev>
Co-committed-by: xbzk <xbzk@eden-emu.dev>

1 files changed, 12 insertions, 7 deletions

diff --git a/src/video_core/buffer_cache/buffer_cache.h b/src/video_core/buffer_cache/buffer_cache.h
index 39c56fb33f..5b5dc1c219 100644
--- a/src/video_core/buffer_cache/buffer_cache.h
+++ b/src/video_core/buffer_cache/buffer_cache.h
@@ -1705,21 +1705,26 @@ Binding BufferCache<P>::StorageBufferBinding(GPUVAddr ssbo_addr, u32 cbuf_index,
         return NULL_BINDING;
     }
 
+    // xbzk: New size logic. Fixes MCI.
+    // If ever the * comment below prove wrong, the 'if' block may be removed.
     const auto size = [&]() {
         const bool is_nvn_cbuf = cbuf_index == 0;
-        // The NVN driver buffer (index 0) is known to pack the SSBO address followed by its size.
         if (is_nvn_cbuf) {
-            const u32 ssbo_size = gpu_memory->Read<u32>(ssbo_addr + 8);
-            if (ssbo_size != 0) {
-                return ssbo_size;
+            // * The NVN driver buffer (index 0) is known to pack the SSBO address followed by its size.
+            const u64 next_qword = gpu_memory->Read<u64>(ssbo_addr + 8);
+            const u32 upper_32 = static_cast<u32>(next_qword >> 32);
+            // Hardware-based detection: GPU addresses have non-zero upper bits
+            if (upper_32 == 0) {
+                // This is a size field, not a GPU address
+                return static_cast<u32>(next_qword);  // Return lower_32
             }
         }
-        // Other titles (notably Doom Eternal) may use STG/LDG on buffer addresses in custom defined
-        // cbufs, which do not store the sizes adjacent to the addresses, so use the fully
-        // mapped buffer size for now.
+        // Fall through: either not NVN cbuf (Doom Eternal & +), or NVN but ssbo_addr+8 is a GPU address (MCI)
         const u32 memory_layout_size = static_cast<u32>(gpu_memory->GetMemoryLayoutSize(gpu_addr));
+        // Cap at 8MB to prevent allocator overflow from misinterpreted addresses
         return (std::min)(memory_layout_size, static_cast<u32>(8_MiB));
     }();
+
     // Alignment only applies to the offset of the buffer
     const u32 alignment = runtime.GetStorageBufferAlignment();
     const GPUVAddr aligned_gpu_addr = Common::AlignDown(gpu_addr, alignment);