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4f510c8bb1
We are dropping A/D bits (and W bits) in the TDP MMU. Even if mmu_lock is held for write, as volatile SPTEs can be written by other tasks/vCPUs outside of mmu_lock. Attempting to prove that bug exposed another notable goof, which has been lurking for a decade, give or take: KVM treats _all_ MMU-writable SPTEs as volatile, even though KVM never clears WRITABLE outside of MMU lock. As a result, the legacy MMU (and the TDP MMU if not fixed) uses XCHG to update writable SPTEs. The fix does not seem to have an easily-measurable affect on performance; page faults are so slow that wasting even a few hundred cycles is dwarfed by the base cost.