The TLAS metadata buffer is CPU-written every frame (one userMetadata word
per element in BuildTLAS's copy loop) and read by the ray shaders as a
STORAGE_BUFFER via device address every frame. Allocated
HOST_VISIBLE | HOST_COHERENT (system RAM), every per-frame shader read
traversed PCIe — the same cost the sibling instance buffer paid before #65.
Upgrade the request to HOST_VISIBLE with a best-effort DEVICE_LOCAL preference
(BAR/VRAM), keeping the single persistently-mapped buffer. The CPU's writes are
write-only, sequential, never-read-back — the ideal write-combined BAR load —
and the shader reads now hit local VRAM.
Unlike the instance buffer there is no GPU co-write, so the direct upgrade is
unconditional. Staging is still deliberately avoided: this buffer is rewritten
by the CPU every frame, so a per-frame stage+copy+barrier would defeat the
purpose (#75 caveat 2).
Correctness:
- DEVICE_LOCAL is a preference only (depends on #59): GetMemoryType falls back
to plain HOST_VISIBLE (the previous behaviour) on GPUs without resizable BAR.
- HOST_COHERENT is dropped from the required set — the combined type may not be
coherent. A FlushDevice() is added after the copy loop to make the host writes
available on a non-coherent type; it self-gates to a no-op on a coherent type
(#60), so the previous behaviour is preserved wherever the type ends up
coherent. The metadata buffer is not an AS build input and has no GPU
co-write, so it needs neither the build-stage barrier the instance buffer
takes nor a HOST->shader command barrier: the queue submit already orders host
writes made available before it against every command in the submission,
exactly as it did when this buffer was HOST_COHERENT.
Extends the TLASHighWaterMark hardware test with a metadata-buffer assertion
block mirroring #65's instance-buffer check: the chosen memory type must equal
what GetMemoryType resolves for the HOST_VISIBLE | prefer-DEVICE_LOCAL request,
and must be DEVICE_LOCAL wherever a host-visible device-local type is reachable.
The zero-validation-errors check covers the dropped HOST_COHERENT path.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
