linux-kernel - [PATCH] KVM: x86/mmu: Wrap sanity check on number of TDP MMU pages with KVM_PROVE

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Message-ID: <20250315023448.2358456-1-seanjc@google.com>
Date: Fri, 14 Mar 2025 19:34:48 -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: [PATCH] KVM: x86/mmu: Wrap sanity check on number of TDP MMU pages
 with KVM_PROVE_MMU

Wrap the TDP MMU page counter in CONFIG_KVM_PROVE_MMU so that the sanity
check is omitted from production builds, and more importantly to remove
the atomic accesses to account pages.  A one-off memory leak in production
is relatively uninteresting, and a WARN_ON won't help mitigate a systemic
issue; it's as much about helping triage memory leaks as it is about
detecting them in the first place, and doesn't magically stop the leaks.
I.e. production environments will be quite sad if a severe KVM bug escapes,
regardless of whether or not KVM WARNs.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/include/asm/kvm_host.h | 7 ++++++-
 arch/x86/kvm/mmu/tdp_mmu.c      | 8 +++++++-
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index d881e7d276b1..30b81a81a1c7 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1471,8 +1471,13 @@ struct kvm_arch {
 	struct once nx_once;
 
 #ifdef CONFIG_X86_64
-	/* The number of TDP MMU pages across all roots. */
+#ifdef CONFIG_KVM_PROVE_MMU
+	/*
+	 * The number of TDP MMU pages across all roots.  Used only to sanity
+	 * check that KVM isn't leaking TDP MMU pages.
+	 */
 	atomic64_t tdp_mmu_pages;
+#endif
 
 	/*
 	 * List of struct kvm_mmu_pages being used as roots.
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 7cc0564f5f97..21a3b8166242 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -40,7 +40,9 @@ void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm)
 	kvm_tdp_mmu_invalidate_roots(kvm, KVM_VALID_ROOTS);
 	kvm_tdp_mmu_zap_invalidated_roots(kvm, false);
 
-	WARN_ON(atomic64_read(&kvm->arch.tdp_mmu_pages));
+#ifdef CONFIG_KVM_PROVE_MMU
+	KVM_MMU_WARN_ON(atomic64_read(&kvm->arch.tdp_mmu_pages));
+#endif
 	WARN_ON(!list_empty(&kvm->arch.tdp_mmu_roots));
 
 	/*
@@ -325,13 +327,17 @@ static void handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
 static void tdp_account_mmu_page(struct kvm *kvm, struct kvm_mmu_page *sp)
 {
 	kvm_account_pgtable_pages((void *)sp->spt, +1);
+#ifdef CONFIG_KVM_PROVE_MMU
 	atomic64_inc(&kvm->arch.tdp_mmu_pages);
+#endif
 }
 
 static void tdp_unaccount_mmu_page(struct kvm *kvm, struct kvm_mmu_page *sp)
 {
 	kvm_account_pgtable_pages((void *)sp->spt, -1);
+#ifdef CONFIG_KVM_PROVE_MMU
 	atomic64_dec(&kvm->arch.tdp_mmu_pages);
+#endif
 }
 
 /**

base-commit: 7d2154117a02832ab3643fe2da4cdc9d2090dcb2
-- 
2.49.0.rc1.451.g8f38331e32-goog