linux-kernel - [PATCH v2 21/22] KVM: x86/mmu: Detect if unprotect will do anything based on invalid

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Message-ID: <20240831001538.336683-22-seanjc@google.com>
Date: Fri, 30 Aug 2024 17:15:36 -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, 
	Yuan Yao <yuan.yao@...el.com>, Yuan Yao <yuan.yao@...ux.intel.com>
Subject: [PATCH v2 21/22] KVM: x86/mmu: Detect if unprotect will do anything
 based on invalid_list

Explicitly query the list of to-be-zapped shadow pages when checking to
see if unprotecting a gfn for retry has succeeded, i.e. if KVM should
retry the faulting instruction.

Add a comment to explain why the list needs to be checked before zapping,
which is the primary motivation for this change.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu/mmu.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index d042874b0a3b..be5c2c33b530 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2721,12 +2721,15 @@ bool __kvm_mmu_unprotect_gfn_and_retry(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 			goto out;
 	}
 
-	r = false;
 	write_lock(&kvm->mmu_lock);
-	for_each_gfn_valid_sp_with_gptes(kvm, sp, gpa_to_gfn(gpa)) {
-		r = true;
+	for_each_gfn_valid_sp_with_gptes(kvm, sp, gpa_to_gfn(gpa))
 		kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list);
-	}
+
+	/*
+	 * Snapshot the result before zapping, as zapping will remove all list
+	 * entries, i.e. checking the list later would yield a false negative.
+	 */
+	r = !list_empty(&invalid_list);
 	kvm_mmu_commit_zap_page(kvm, &invalid_list);
 	write_unlock(&kvm->mmu_lock);
 
-- 
2.46.0.469.g59c65b2a67-goog