linux-kernel - [RFC PATCH 4/9] KVM: x86/mmu: Use Accessed bit even when _hardware

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Message-ID: <20240801183453.57199-5-seanjc@google.com>
Date: Thu,  1 Aug 2024 11:34:48 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [RFC PATCH 4/9] 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.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu/mmu.c  |  6 ++++--
 arch/x86/kvm/mmu/spte.c |  6 +++---
 arch/x86/kvm/mmu/spte.h | 11 +----------
 3 files changed, 8 insertions(+), 15 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 1e24bc4a06db..c8fc59fcc8e0 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3454,8 +3454,10 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 		 * uses A/D bits for non-nested MMUs.  Thus, if A/D bits are
 		 * 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);
+		if (unlikely(!kvm_ad_enabled) && is_access_track_spte(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 a0ff504f1e7e..ca1a8116de34 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -181,7 +181,7 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 
 	spte |= shadow_present_mask;
 	if (!prefetch)
-		spte |= spte_shadow_accessed_mask(spte);
+		spte |= shadow_accessed_mask;
 
 	/*
 	 * For simplicity, enforce the NX huge page mitigation even if not
@@ -258,7 +258,7 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 	}
 
 	if (pte_access & ACC_WRITE_MASK)
-		spte |= spte_shadow_dirty_mask(spte);
+		spte |= shadow_accessed_mask;
 
 out:
 	if (prefetch)
@@ -367,7 +367,7 @@ 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 d722b37b7434..ba7ff1dfbeb2 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 bool is_dirty_spte(u64 spte)
-- 
2.46.0.rc1.232.g9752f9e123-goog