KVM: x86/mmu: Don't force atomic update if only the Accessed bit is volatile

KVM: x86/mmu: Don't force atomic update if only the Accessed bit is volatile

Don't force SPTE modifications to be done atomically if the only volatile
bit in the SPTE is the Accessed bit.  KVM and the primary MMU tolerate
stale aging state, and the probability of an Accessed bit A/D assist being
clobbered *and* affecting again is likely far lower than the probability
of consuming stale information due to not flushing TLBs when aging.

Rename spte_has_volatile_bits() to spte_needs_atomic_update() to better
capture the nature of the helper.

Opportunstically do s/write/update on the TDP MMU wrapper, as it's not
simply the "write" that needs to be done atomically, it's the entire
update, i.e. the entire read-modify-write operation needs to be done
atomically so that KVM has an accurate view of the old SPTE.

Leave kvm_tdp_mmu_write_spte_atomic() as is.  While the name is imperfect,
it pairs with kvm_tdp_mmu_write_spte(), which in turn pairs with
kvm_tdp_mmu_read_spte().  And renaming all of those isn't obviously a net
positive, and would require significant churn.

Signed-off-by: James Houghton <jthoughton@google.com>
Link: https://lore.kernel.org/r/20250204004038.1680123-6-jthoughton@google.com
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>