KVM: x86/mmu: Add comment on try_cmpxchg64 usage in tdp_mmu_set_spte_atomic

Commit aee98a68 ("KVM: x86/mmu: Use try_cmpxchg64 in
tdp_mmu_set_spte_atomic") removed the comment that iter->old_spte is
updated when different logical CPU modifies the page table entry.
Although this is what try_cmpxchg does implicitly, it won't hurt
if this fact is explicitly mentioned in a restored comment.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: David Matlack <dmatlack@google.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Link: https://lore.kernel.org/r/20230425113932.3148-1-ubizjak@gmail.com
[sean: extend comment above try_cmpxchg64()]
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/mmu/tdp_mmu.c

@@ -592,7 +592,10 @@ static inline int tdp_mmu_set_spte_atomic(struct kvm *kvm,
 
 	/*
 	 * Note, fast_pf_fix_direct_spte() can also modify TDP MMU SPTEs and
-	 * does not hold the mmu_lock.
+	 * does not hold the mmu_lock.  On failure, i.e. if a different logical
+	 * CPU modified the SPTE, try_cmpxchg64() updates iter->old_spte with
+	 * the current value, so the caller operates on fresh data, e.g. if it
+	 * retries tdp_mmu_set_spte_atomic()
 	 */
 	if (!try_cmpxchg64(sptep, &iter->old_spte, new_spte))
 		return -EBUSY;