On Linux, shared direct memory is imported through /dev/udmabuf (#943).
Many distributions create it as root:kvm 0660, so a user outside the
kvm group cannot open it. The ranges are then copied instead of
imported, and GPU stores through FLAT/GLOBAL addresses to them fail
with "BDA access failed": DOOM PPSA99666's compute presenter falls back
to its CPU scaler. The only trace was "failed at open /dev/udmabuf
(-1000072003)", a Vulkan result that hides errno.
Print the open error once, with the fix for EACCES, and document the
requirement in USAGE.md.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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