KVM: x86/mmu: Atomically clear SPTE dirty state in the clear-dirty-log flow

Optimize the clearing of dirty state in TDP MMU SPTEs by doing an
atomic-AND (on SPTEs that have volatile bits) instead of the full XCHG
that currently ends up being invoked (see kvm_tdp_mmu_write_spte()).
Clearing _only_ the bit in question will allow KVM to skip the many
irrelevant checks in __handle_changed_spte() by avoiding any collateral
damage due to the XCHG writing all SPTE bits, e.g. the XCHG could race
with fast_page_fault() setting the W-bit and the CPU setting the D-bit,
and thus incorrectly drop the CPU's D-bit update.

Link: https://lore.kernel.org/all/Y9hXmz%[email protected]
Signed-off-by: Vipin Sharma <[email protected]>
Reviewed-by: David Matlack <[email protected]>
[sean: split the switch to atomic-AND to a separate patch]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

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