[PATCH 12/18] KVM: x86/mmu: Use Accessed bit even when _hardware_ A/D bits are disabled

Sean Christopherson
2024-10-11T02:11:17Z

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.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/mmu/mmu.c | 3 ++-
arch/x86/kvm/mmu/spte.c | 4 ++--
arch/x86/kvm/mmu/spte.h | 11 +----------
3 files changed, 5 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 06fb0fd3f87d..5be3b5f054f1 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3486,7 +3486,8 @@ 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 54d8c9b76050..617479efd127 100644
--- 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 908cb672c4e1..c8dc75337c8b 100644
--- 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,
--
2.47.0.rc1.288.g06298d1525-goog