The shader binding table is written once at pipeline creation and read by
the GPU on every vkCmdTraceRaysKHR thereafter, yet it was allocated plain
HOST_VISIBLE (system RAM) — so every trace dispatch read raygen/miss/hit
records over PCIe.
Route the SBT allocation through Device::PreferDirectDeviceWrite (#89):
when a DEVICE_LOCAL|HOST_VISIBLE type exists, prefer DEVICE_LOCAL on top
of the required HOST_VISIBLE so the buffer lands in mappable device
memory (GPU reads hit local VRAM; the one-time memcpy goes write-combined
over PCIe). When no combined type exists, the preferred hint is dropped
and GetMemoryType (#59) falls back to plain HOST_VISIBLE — current
behaviour. The SBT is tiny so a small BAR window's budget is never the
constraint.
The buffer stays mapped and the existing FlushDevice gates on the chosen
memory type's coherency flags (#60), so a direct-write type lacking
HOST_COHERENT is still flushed correctly.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
