linux-kernel - [PATCH 1/4] KVM: x86/mmu: Further check old SPTE is leaf for spurious prefetch fault

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Message-ID: <20250207030739.1649-1-yan.y.zhao@intel.com>
Date: Fri,  7 Feb 2025 11:07:39 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: pbonzini@...hat.com,
	seanjc@...gle.com
Cc: rick.p.edgecombe@...el.com,
	linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org,
	Yan Zhao <yan.y.zhao@...el.com>
Subject: [PATCH 1/4] KVM: x86/mmu: Further check old SPTE is leaf for spurious prefetch fault

Instead of simply treating a prefetch fault as spurious when there's a
shadow-present old SPTE, further check if the old SPTE is leaf to determine
if a prefetch fault is spurious.

It's not reasonable to treat a prefetch fault as spurious when there's a
shadow-present non-leaf SPTE while without a shadow-present leaf SPTE.
e.g., with below sequence, a prefetch fault should not be regarded as
spurious:
1. add a memslot with size 4K
2. prefault GPA A in the memslot
3. delete the memslot (zap all disabled)
4. re-add the memslot with size 2M
5. prefault GPA A again.
In step 5, the prefetch fault attempts to install a 2M huge entry.
Since step 3 zaps the leaf SPTE for GPA A while keeping the non-leaf SPTE,
the leaf entry will remain empty after step 5 if the prefetch fault is
regarded as spurious due to a shadow-present non-leaf SPTE found.

Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
---
 arch/x86/kvm/mmu/mmu.c     | 2 +-
 arch/x86/kvm/mmu/tdp_mmu.c | 3 ++-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index a45ae60e84ab..3d74e680006f 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2846,7 +2846,7 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
 	}

 	if (is_shadow_present_pte(*sptep)) {
-		if (prefetch)
+		if (prefetch && is_last_spte(*sptep, level))
 			return RET_PF_SPURIOUS;

 		/*
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 046b6ba31197..ab65fd915ef2 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1137,7 +1137,8 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
 	if (WARN_ON_ONCE(sp->role.level != fault->goal_level))
 		return RET_PF_RETRY;

-	if (fault->prefetch && is_shadow_present_pte(iter->old_spte))
+	if (fault->prefetch && is_shadow_present_pte(iter->old_spte) &&
+	    is_last_spte(iter->old_spte, iter->level))
 		return RET_PF_SPURIOUS;

 	if (is_shadow_present_pte(iter->old_spte) &&
-- 
2.43.2