KVM: x86/mmu: Use CMPXCHG when clearing Accessed bit in the shadow MMU

Use CMPXCHG instead of clear_bit(), which currently emits a LOCK BTR since
the to-be-cleared bit isn't a compile-time constant, when aging SPTEs in
the shadow MMU to align with the approach taken by the TDP MMU, and because
using CMPXCHG is far more robust against bugs in KVM.  E.g. if the SPTE is
somehow no longer an SPTE due to a KVM bug, CMPXCHG will fail gracefully,
whereas clear_bit() would potentially corrupt/clobber memory.

Clearing the Accessed bit without atomically ensuring the SPTE is still the
old SPTE is "fine", as holding the rmap's lock ensures zapping the old SPTE
can't fully complete, which in turn ensures a new, different SPTE can't be
installed.  But that chain of logic isn't exactly obvious, and there's zero
reason to avoid CMPXCHG as its cost on modern hardware is within ~1-2 uops
of LOCK BTR (and may even be cheaper on some microarchitectures).  Doing a
64-bit CMPXCHG on 32-bit kernels does require a more expensive CMPXCHG8B,
but 32-bit KVM is all but dead at this point.

Cc: James Houghton <jthoughton@google.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: James Houghton <jthoughton@google.com>
Link: https://patch.msgid.link/20260728002236.869865-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/mmu/mmu.c

@@ -1718,38 +1718,39 @@ static bool kvm_rmap_age_gfn_range(struct kvm *kvm,
 	struct kvm_rmap_head *rmap_head;
 	struct rmap_iterator iter;
 	unsigned long rmap_val;
+	u64 old_spte, new_spte;
 	bool young = false;
 	u64 *sptep;
 	gfn_t gfn;
 	int level;
-	u64 spte;
 
 	for (level = PG_LEVEL_4K; level <= KVM_MAX_HUGEPAGE_LEVEL; level++) {
 		for (gfn = range->start; gfn < range->end;
 		     gfn += KVM_PAGES_PER_HPAGE(level)) {
 			rmap_head = gfn_to_rmap(gfn, level, range->slot);
 			rmap_val = kvm_rmap_lock_readonly(rmap_head);
 
-			for_each_rmap_spte_lockless(rmap_val, &iter, sptep, spte) {
-				if (!is_accessed_spte(spte))
+			for_each_rmap_spte_lockless(rmap_val, &iter, sptep, old_spte) {
+				if (!is_accessed_spte(old_spte))
 					continue;
 
 				if (test_only) {
 					kvm_rmap_unlock_readonly(rmap_head, rmap_val);
 					return true;
 				}
 
-				if (spte_ad_enabled(spte))
-					clear_bit((ffs(shadow_accessed_mask) - 1),
-						  (unsigned long *)sptep);
+				if (spte_ad_enabled(old_spte))
+					new_spte = old_spte & ~shadow_accessed_mask;
 				else
-					/*
-					 * If the following cmpxchg fails, the
-					 * spte is being concurrently modified
-					 * and should most likely stay young.
-					 */
-					cmpxchg64(sptep, spte,
-					      mark_spte_for_access_track(spte));
+					new_spte = mark_spte_for_access_track(old_spte);
+
+				/*
+				 * Don't bother retrying if the CMPXCHG fails,
+				 * i.e. if another CPU modified the SPTE.  The
+				 * SPTE is either being zapped or is likely
+				 * still in-use, i.e. is still young.
+				 */
+				cmpxchg64(sptep, old_spte, new_spte);
 				young = true;
 			}