KVM: x86/mmu: Use Accessed bit even when _hardware_ A/D bits are disabled

Use the Accessed bit in SPTEs even when A/D bits are disabled in hardware,
i.e. propagate accessed information to SPTE.Accessed even when KVM is
doing manual tracking by making SPTEs not-present.  In addition to
eliminating a small amount of code in is_accessed_spte(), this also paves
the way for preserving Accessed information when a SPTE is zapped in
response to a mmu_notifier PROTECTION event, e.g. if a SPTE is zapped
because NUMA balancing kicks in.

Note, EPT is the only flavor of paging in which A/D bits are conditionally
enabled, and the Accessed (and Dirty) bit is software-available when A/D
bits are disabled.

Note #2, there are currently no concrete plans to preserve Accessed
information.  Explorations on that front were the initial catalyst, but
the cleanup is the motivation for the actual commit.

Link: https://lore.kernel.org/r/20241011021051.1557902-13-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/mmu/mmu.c

@@ -3493,7 +3493,8 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 		 * enabled, the SPTE can't be an access-tracked SPTE.
 		 */
 		if (unlikely(!kvm_ad_enabled) && is_access_track_spte(spte))
-			new_spte = restore_acc_track_spte(new_spte);
+			new_spte = restore_acc_track_spte(new_spte) |
+				   shadow_accessed_mask;
 
 		/*
 		 * To keep things simple, only SPTEs that are MMU-writable can

arch/x86/kvm/mmu/spte.c

@@ -175,7 +175,7 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 
 	spte |= shadow_present_mask;
 	if (!prefetch || synchronizing)
-		spte |= spte_shadow_accessed_mask(spte);
+		spte |= shadow_accessed_mask;
 
 	/*
 	 * For simplicity, enforce the NX huge page mitigation even if not
@@ -346,7 +346,7 @@ u64 mark_spte_for_access_track(u64 spte)
 
 	spte |= (spte & SHADOW_ACC_TRACK_SAVED_BITS_MASK) <<
 		SHADOW_ACC_TRACK_SAVED_BITS_SHIFT;
-	spte &= ~shadow_acc_track_mask;
+	spte &= ~(shadow_acc_track_mask | shadow_accessed_mask);
 
 	return spte;
 }

arch/x86/kvm/mmu/spte.h

@@ -316,12 +316,6 @@ static inline bool spte_ad_need_write_protect(u64 spte)
 	return (spte & SPTE_TDP_AD_MASK) != SPTE_TDP_AD_ENABLED;
 }
 
-static inline u64 spte_shadow_accessed_mask(u64 spte)
-{
-	KVM_MMU_WARN_ON(!is_shadow_present_pte(spte));
-	return spte_ad_enabled(spte) ? shadow_accessed_mask : 0;
-}
-
 static inline u64 spte_shadow_dirty_mask(u64 spte)
 {
 	KVM_MMU_WARN_ON(!is_shadow_present_pte(spte));
@@ -355,10 +349,7 @@ static inline kvm_pfn_t spte_to_pfn(u64 pte)
 
 static inline bool is_accessed_spte(u64 spte)
 {
-	u64 accessed_mask = spte_shadow_accessed_mask(spte);
-
-	return accessed_mask ? spte & accessed_mask
-			     : !is_access_track_spte(spte);
+	return spte & shadow_accessed_mask;
 }
 
 static inline u64 get_rsvd_bits(struct rsvd_bits_validate *rsvd_check, u64 pte,