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>

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 80b5a45ac3178f7d64f7a4d11acb732be5d647bc..36247b8d747671bb31813a3b0f34ed5768113158 100644 (file)
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/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

diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index 54d8c9b76050606e65da57f2bd9134abaf78c1f3..617479efd127a9ba261baa78bc0273b32e3a7a63 100644 (file)
--- a/arch/x86/kvm/mmu/spte.c
+++ b/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;
 }

diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index 908cb672c4e1994b6954e4a05372606f38c00871..c8dc75337c8ba8defeb592175411f0b474a4e8b7 100644 (file)
--- a/arch/x86/kvm/mmu/spte.h
+++ b/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,