[vfio-users] VFIO and KSM (and maybe hugepages)

[Colin Godsey]

I had a few questions regarding general ‘page management’ and VFIO, mostly
related to kernel shared pages.

I have a host running 2 virtual ‘gaming rigs’ with a single dedicated GPU
each. I had an intermittent problem where when gaming (on the same game)
with both rigs, one would receive graphic artifacts. Specifically I would
see triangle/geometry artifacts which usually indicate corrupt GPU RAM.

Both cards are so completely different, and different generation, one is
really new, I didn’t believe it was bad VRAM. Graphics drivers to swap
various buffers from system RAM to VRAM so I figured it could also be
something related to system RAM.

I disabled any kind of… alternative page management I could- swap, KSM,
huge pages etc. and it did fix it. Because the issue only would effect one
machine, and I only observed it when the same game was running on both, I
assumed maybe it was related to KSM.

*Is there any possible way KSM could interfere with the DMAR in some way
where it tries to share/alter DMA regions?* And broader: what prevents
systems like khugepage, kswap, and ksm from interfering with these regions
in the first place? I’ve read that transparent hugepages can interfere with
VFIO, is it safe to assume that other DMA issues could arise with other
types of page management?
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