interfaces/Crafter.Graphics-PipelineRTVulkan.cppm

@@ -103,7 +103,30 @@ export namespace Crafter {
             hitRegion.size = hitGroups.size() * sbtStride;
 
             std::size_t bufferSize =  hitRegion.deviceAddress + hitRegion.size;
-            sbtBuffer.Create(VK_BUFFER_USAGE_2_SHADER_BINDING_TABLE_BIT_KHR | VK_BUFFER_USAGE_SHADER_DEVICE_ADDRESS_BIT, VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT, bufferSize);
+            // The SBT is written once here (the memcpys below) and read by the
+            // GPU on every vkCmdTraceRaysKHR for the pipeline's lifetime — the
+            // textbook write-once/read-many buffer (issue #72). Get it into
+            // device-local memory so trace dispatches read raygen/miss/hit
+            // records out of VRAM instead of over PCIe from system RAM.
+            //
+            // Route the placement through #89: when a DEVICE_LOCAL|HOST_VISIBLE
+            // type exists (ReBAR/UMA, or a BAR window — the SBT is tiny so the
+            // window budget is never the constraint), prefer DEVICE_LOCAL on top
+            // of the required HOST_VISIBLE so the allocation lands in device
+            // memory we can still map. The one-time memcpy goes write-combined
+            // over PCIe (write-only, sequential — ideal); GPU reads then hit
+            // local VRAM. When no combined type exists at all (no spec
+            // guarantee), PreferDirectDeviceWrite returns false and the preferred
+            // hint is dropped, so GetMemoryType falls back to plain HOST_VISIBLE
+            // (current behaviour, the per-trace PCIe read) — the cheaper fallback
+            // the issue blesses while staging isn't wired here.
+            //
+            // Either way the buffer stays mapped, and the FlushDevice below gates
+            // its flush on the *chosen* memory type's flags (issue #60), so a
+            // direct-write type lacking HOST_COHERENT is still flushed correctly.
+            VkMemoryPropertyFlags sbtPreferred = Device::PreferDirectDeviceWrite(bufferSize)
+                ? VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT : 0;
+            sbtBuffer.Create(VK_BUFFER_USAGE_2_SHADER_BINDING_TABLE_BIT_KHR | VK_BUFFER_USAGE_SHADER_DEVICE_ADDRESS_BIT, VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT, bufferSize, sbtPreferred);
 
             std::uint8_t* offset = sbtBuffer.value;
             std::uint8_t* handleOffset = shaderHandles.data();