linux-kernel - [PATCH] KVM: SVM: Fix redundant updates of LBR MSR intercepts

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Message-ID: <20251112013017.1836863-1-yosry.ahmed@linux.dev>
Date: Wed, 12 Nov 2025 01:30:17 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Paolo Bonzini <pbonzini@...hat.com>,
	Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Yosry Ahmed <yosry.ahmed@...ux.dev>,
	stable@...r.kernel.org
Subject: [PATCH] KVM: SVM: Fix redundant updates of LBR MSR intercepts

svm_update_lbrv() always updates LBR MSRs intercepts, even when they are
already set correctly. This results in force_msr_bitmap_recalc always
being set to true on every nested transition, essentially undoing the
hyperv optimization in nested_svm_merge_msrpm().

Fix it by keeping track of whether LBR MSRs are intercepted or not and
only doing the update if needed, similar to x2avic_msrs_intercepted.

Avoid using svm_test_msr_bitmap_*() to check the status of the
intercepts, as an arbitrary MSR will need to be chosen as a
representative of all LBR MSRs, and this could theoretically break if
some of the MSRs intercepts are handled differently from the rest.

Also, using svm_test_msr_bitmap_*() makes backports difficult as it was
only recently introduced with no direct alternatives in older kernels.

Fixes: fbe5e5f030c2 ("KVM: nSVM: Always recalculate LBR MSR intercepts in svm_update_lbrv()")
Cc: stable@...r.kernel.org
Signed-off-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
---
 arch/x86/kvm/svm/svm.c | 9 ++++++++-
 arch/x86/kvm/svm/svm.h | 1 +
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 10c21e4c5406f..9d29b2e7e855d 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -705,7 +705,11 @@ void *svm_alloc_permissions_map(unsigned long size, gfp_t gfp_mask)
 
 static void svm_recalc_lbr_msr_intercepts(struct kvm_vcpu *vcpu)
 {
-	bool intercept = !(to_svm(vcpu)->vmcb->control.virt_ext & LBR_CTL_ENABLE_MASK);
+	struct vcpu_svm *svm = to_svm(vcpu);
+	bool intercept = !(svm->vmcb->control.virt_ext & LBR_CTL_ENABLE_MASK);
+
+	if (intercept == svm->lbr_msrs_intercepted)
+		return;
 
 	svm_set_intercept_for_msr(vcpu, MSR_IA32_LASTBRANCHFROMIP, MSR_TYPE_RW, intercept);
 	svm_set_intercept_for_msr(vcpu, MSR_IA32_LASTBRANCHTOIP, MSR_TYPE_RW, intercept);
@@ -714,6 +718,8 @@ static void svm_recalc_lbr_msr_intercepts(struct kvm_vcpu *vcpu)
 
 	if (sev_es_guest(vcpu->kvm))
 		svm_set_intercept_for_msr(vcpu, MSR_IA32_DEBUGCTLMSR, MSR_TYPE_RW, intercept);
+
+	svm->lbr_msrs_intercepted = intercept;
 }
 
 void svm_vcpu_free_msrpm(void *msrpm)
@@ -1221,6 +1227,7 @@ static int svm_vcpu_create(struct kvm_vcpu *vcpu)
 	}
 
 	svm->x2avic_msrs_intercepted = true;
+	svm->lbr_msrs_intercepted = true;
 
 	svm->vmcb01.ptr = page_address(vmcb01_page);
 	svm->vmcb01.pa = __sme_set(page_to_pfn(vmcb01_page) << PAGE_SHIFT);
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index c856d8e0f95e7..dd78e64023450 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -336,6 +336,7 @@ struct vcpu_svm {
 	bool guest_state_loaded;
 
 	bool x2avic_msrs_intercepted;
+	bool lbr_msrs_intercepted;
 
 	/* Guest GIF value, used when vGIF is not enabled */
 	bool guest_gif;

base-commit: 8a4821412cf2c1429fffa07c012dd150f2edf78c
-- 
2.51.2.1041.gc1ab5b90ca-goog