FAILED: patch "[PATCH] KVM: nSVM: Fix and simplify LBR virtualization handling with" failed to apply to 6.6-stable tree

PATCH New [email protected] 2 participants 5 messages Nov 20, 2025 – Dec 3, 2025

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Stable branch x86 KVM x86

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author maintainer

2025-11-20 15:51 UTC #0 lore raw

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 8a4821412cf2c1429fffa07c012dd150f2edf78c
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025112059-labrador-contently-dd98@gregkh' --subject-prefix 'PATCH 6.6.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 8a4821412cf2c1429fffa07c012dd150f2edf78c Mon Sep 17 00:00:00 2001
From: Yosry Ahmed <[email protected]>
Date: Sat, 8 Nov 2025 00:45:21 +0000
Subject: [PATCH] KVM: nSVM: Fix and simplify LBR virtualization handling with
nested

The current scheme for handling LBRV when nested is used is very complicated, especially when L1 does not enable LBRV (i.e. does not set LBR_CTL_ENABLE_MASK).

To avoid copying LBRs between VMCB01 and VMCB02 on every nested transition, the current implementation switches between using VMCB01 or VMCB02 as the source of truth for the LBRs while L2 is running. If L2 enables LBR, VMCB02 is used as the source of truth. When L2 disables LBR, the LBRs are copied to VMCB01 and VMCB01 is used as the source of truth. This introduces significant complexity, and incorrect behavior in some cases.

For example, on a nested #VMEXIT, the LBRs are only copied from VMCB02 to VMCB01 if LBRV is enabled in VMCB01. This is because L2's writes to MSR_IA32_DEBUGCTLMSR to enable LBR are intercepted and propagated to VMCB01 instead of VMCB02. However, LBRV is only enabled in VMCB02 when L2 is running.

This means that if L2 enables LBR and exits to L1, the LBRs will not be propagated from VMCB02 to VMCB01, because LBRV is disabled in VMCB01.

There is no meaningful difference in CPUID rate in L2 when copying LBRs on every nested transition vs. the current approach, so do the simple and correct thing and always copy LBRs between VMCB01 and VMCB02 on nested transitions (when LBRV is disabled by L1). Drop the conditional LBRs copying in __svm_{enable/disable}_lbrv() as it is now unnecessary.

VMCB02 becomes the only source of truth for LBRs when L2 is running, regardless of LBRV being enabled by L1, drop svm_get_lbr_vmcb() and use svm->vmcb directly in its place.

Fixes 1d5a1b5860ed ("KVM: x86: nSVM: correctly virtualize LBR msrs when L2 is running")

Cc [email protected]

Signed-off-by Yosry Ahmed <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Signed-off-by Paolo Bonzini <[email protected]>

arch/x86/kvm/svm/nested.c +7 ~~-13~~

index a6443feab252..da6e80b3ac35 100644
@@ -677,11 +677,10 @@ static void nested_vmcb02_prepare_save(struct vcpu_svm *svm, struct vmcb *vmcb12
 		 */
 		svm_copy_lbrs(vmcb02, vmcb12);
 		vmcb02->save.dbgctl &= ~DEBUGCTL_RESERVED_BITS;
-		svm_update_lbrv(&svm->vcpu);
-
-	} else if (unlikely(vmcb01->control.virt_ext & LBR_CTL_ENABLE_MASK)) {
+	} else {
 		svm_copy_lbrs(vmcb02, vmcb01);
 	}
+	svm_update_lbrv(&svm->vcpu);
 }
 
 static inline bool is_evtinj_soft(u32 evtinj)

@@ -833,11 +832,7 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
 			svm->soft_int_next_rip = vmcb12_rip;
 	}
 
-	vmcb02->control.virt_ext            = vmcb01->control.virt_ext &
-					      LBR_CTL_ENABLE_MASK;
-	if (guest_cpu_cap_has(vcpu, X86_FEATURE_LBRV))
-		vmcb02->control.virt_ext  |=
-			(svm->nested.ctl.virt_ext & LBR_CTL_ENABLE_MASK);
+	/* LBR_CTL_ENABLE_MASK is controlled by svm_update_lbrv() */
 
 	if (!nested_vmcb_needs_vls_intercept(svm))
 		vmcb02->control.virt_ext |= VIRTUAL_VMLOAD_VMSAVE_ENABLE_MASK;

@@ -1189,13 +1184,12 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
 		kvm_make_request(KVM_REQ_EVENT, &svm->vcpu);
 
 	if (unlikely(guest_cpu_cap_has(vcpu, X86_FEATURE_LBRV) &&
-		     (svm->nested.ctl.virt_ext & LBR_CTL_ENABLE_MASK))) {
+		     (svm->nested.ctl.virt_ext & LBR_CTL_ENABLE_MASK)))
 		svm_copy_lbrs(vmcb12, vmcb02);
-		svm_update_lbrv(vcpu);
-	} else if (unlikely(vmcb01->control.virt_ext & LBR_CTL_ENABLE_MASK)) {
+	else
 		svm_copy_lbrs(vmcb01, vmcb02);
-		svm_update_lbrv(vcpu);
-	}
+
+	svm_update_lbrv(vcpu);
 
 	if (vnmi) {
 		if (vmcb02->control.int_ctl & V_NMI_BLOCKING_MASK)

arch/x86/kvm/svm/svm.c +10 ~~-36~~

87 lines not on this page · raw message

Yosry Ahmed

2025-12-03 18:42 UTC #1 lore raw

PATCH 1/4 KVM: SVM: Introduce svm_recalc_lbr_msr_intercepts()

Introduce a helper updating the intercepts for LBR MSRs, similar to the one introduced upstream by commit 160f143cc131 ("KVM: SVM: Manually recalc all MSR intercepts on userspace MSR filter change"). The main difference is that this version uses set_msr_interception(), which has inverted polarity compared to svm_set_intercept_for_msr().

This is intended to simplify incoming backports. No functional changes intended.

Signed-off-by Yosry Ahmed <[email protected]>

---
 arch/x86/kvm/svm/svm.c | 32 +++++++++++++++++++++-----------
 1 file changed, 21 insertions(+), 11 deletions(-)

arch/x86/kvm/svm/svm.c +21 ~~-11~~

index 99180d0af3ea..d0bfcf737512 100644
@@ -1009,18 +1009,31 @@ void svm_copy_lbrs(struct vmcb *to_vmcb, struct vmcb *from_vmcb)
 	vmcb_mark_dirty(to_vmcb, VMCB_LBR);
 }
 
-void svm_enable_lbrv(struct kvm_vcpu *vcpu)
+static void svm_recalc_lbr_msr_intercepts(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
+	bool intercept = !(svm->vmcb->control.virt_ext & LBR_CTL_ENABLE_MASK);
 
-	svm->vmcb->control.virt_ext |= LBR_CTL_ENABLE_MASK;
-	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_LASTBRANCHFROMIP, 1, 1);
-	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_LASTBRANCHTOIP, 1, 1);
-	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_LASTINTFROMIP, 1, 1);
-	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_LASTINTTOIP, 1, 1);
+	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_LASTBRANCHFROMIP,
+			     !intercept, !intercept);
+	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_LASTBRANCHTOIP,
+			     !intercept, !intercept);
+	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_LASTINTFROMIP,
+			     !intercept, !intercept);
+	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_LASTINTTOIP,
+			     !intercept, !intercept);
 
 	if (sev_es_guest(vcpu->kvm))
-		set_msr_interception(vcpu, svm->msrpm, MSR_IA32_DEBUGCTLMSR, 1, 1);
+		set_msr_interception(vcpu, svm->msrpm, MSR_IA32_DEBUGCTLMSR,
+				     !intercept, !intercept);
+}
+
+void svm_enable_lbrv(struct kvm_vcpu *vcpu)
+{
+	struct vcpu_svm *svm = to_svm(vcpu);
+
+	svm->vmcb->control.virt_ext |= LBR_CTL_ENABLE_MASK;
+	svm_recalc_lbr_msr_intercepts(vcpu);
 
 	/* Move the LBR msrs to the vmcb02 so that the guest can see them. */
 	if (is_guest_mode(vcpu))

@@ -1034,10 +1047,7 @@ static void svm_disable_lbrv(struct kvm_vcpu *vcpu)
 	KVM_BUG_ON(sev_es_guest(vcpu->kvm), vcpu->kvm);
 
 	svm->vmcb->control.virt_ext &= ~LBR_CTL_ENABLE_MASK;
-	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_LASTBRANCHFROMIP, 0, 0);
-	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_LASTBRANCHTOIP, 0, 0);
-	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_LASTINTFROMIP, 0, 0);
-	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_LASTINTTOIP, 0, 0);
+	svm_recalc_lbr_msr_intercepts(vcpu);
 
 	/*
 	 * Move the LBR msrs back to the vmcb01 to avoid copying them

Signature · 1 line

2.52.0.158.g65b55ccf14-goog

Yosry Ahmed

2025-12-03 18:42 UTC #2 lore raw

PATCH 2/4 KVM: nSVM: Always recalculate LBR MSR intercepts in svm_update_lbrv()

svm_update_lbrv() is called when MSR_IA32_DEBUGCTLMSR is updated, and on nested transitions where LBRV is used. It checks whether LBRV enablement needs to be changed in the current VMCB, and if it does, it also recalculate intercepts to LBR MSRs.

However, there are cases where intercepts need to be updated even when LBRV enablement doesn't. Example scenario:
- L1 has MSR_IA32_DEBUGCTLMSR cleared.
- L1 runs L2 without LBR_CTL_ENABLE (no LBRV).
- L2 sets DEBUGCTLMSR_LBR in MSR_IA32_DEBUGCTLMSR, svm_update_lbrv() sets LBR_CTL_ENABLE in VMCB02 and disables intercepts to LBR MSRs.
- L2 exits to L1, svm_update_lbrv() is not called on this transition.
- L1 clears MSR_IA32_DEBUGCTLMSR, svm_update_lbrv() finds that LBR_CTL_ENABLE is already cleared in VMCB01 and does nothing.
- Intercepts remain disabled, L1 reads to LBR MSRs read the host MSRs.

Fix it by always recalculating intercepts in svm_update_lbrv().

Fixes 1d5a1b5860ed ("KVM: x86: nSVM: correctly virtualize LBR msrs when L2 is running")

Cc [email protected]

Signed-off-by Yosry Ahmed <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Signed-off-by Paolo Bonzini <[email protected]>

(cherry picked from commit fbe5e5f030c22ae717ee422aaab0e00ea84fab5e)

Signed-off-by Yosry Ahmed <[email protected]>

---
 arch/x86/kvm/svm/svm.c | 29 +++++++++++++++++++----------
 1 file changed, 19 insertions(+), 10 deletions(-)

arch/x86/kvm/svm/svm.c +19 ~~-10~~

index d0bfcf737512..9254e662c78c 100644
@@ -1028,26 +1028,30 @@ static void svm_recalc_lbr_msr_intercepts(struct kvm_vcpu *vcpu)
 				     !intercept, !intercept);
 }
 
-void svm_enable_lbrv(struct kvm_vcpu *vcpu)
+static void __svm_enable_lbrv(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
 
 	svm->vmcb->control.virt_ext |= LBR_CTL_ENABLE_MASK;
-	svm_recalc_lbr_msr_intercepts(vcpu);
 
 	/* Move the LBR msrs to the vmcb02 so that the guest can see them. */
 	if (is_guest_mode(vcpu))
 		svm_copy_lbrs(svm->vmcb, svm->vmcb01.ptr);
 }
 
-static void svm_disable_lbrv(struct kvm_vcpu *vcpu)
+void svm_enable_lbrv(struct kvm_vcpu *vcpu)
+{
+	__svm_enable_lbrv(vcpu);
+	svm_recalc_lbr_msr_intercepts(vcpu);
+}
+
+static void __svm_disable_lbrv(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
 
 	KVM_BUG_ON(sev_es_guest(vcpu->kvm), vcpu->kvm);
 
 	svm->vmcb->control.virt_ext &= ~LBR_CTL_ENABLE_MASK;
-	svm_recalc_lbr_msr_intercepts(vcpu);
 
 	/*
 	 * Move the LBR msrs back to the vmcb01 to avoid copying them

@@ -1076,13 +1080,18 @@ void svm_update_lbrv(struct kvm_vcpu *vcpu)
 			    (is_guest_mode(vcpu) && guest_can_use(vcpu, X86_FEATURE_LBRV) &&
 			    (svm->nested.ctl.virt_ext & LBR_CTL_ENABLE_MASK));
 
-	if (enable_lbrv == current_enable_lbrv)
-		return;
+	if (enable_lbrv && !current_enable_lbrv)
+		__svm_enable_lbrv(vcpu);
+	else if (!enable_lbrv && current_enable_lbrv)
+		__svm_disable_lbrv(vcpu);
 
-	if (enable_lbrv)
-		svm_enable_lbrv(vcpu);
-	else
-		svm_disable_lbrv(vcpu);
+	/*
+	 * During nested transitions, it is possible that the current VMCB has
+	 * LBR_CTL set, but the previous LBR_CTL had it cleared (or vice versa).
+	 * In this case, even though LBR_CTL does not need an update, intercepts
+	 * do, so always recalculate the intercepts here.
+	 */
+	svm_recalc_lbr_msr_intercepts(vcpu);
 }
 
 void disable_nmi_singlestep(struct vcpu_svm *svm)

Signature · 1 line

2.52.0.158.g65b55ccf14-goog

Yosry Ahmed

2025-12-03 18:42 UTC #3 lore raw

PATCH 3/4 KVM: nSVM: Fix and simplify LBR virtualization handling with nested

In reply to Yosry Ahmed

The current scheme for handling LBRV when nested is used is very complicated, especially when L1 does not enable LBRV (i.e. does not set LBR_CTL_ENABLE_MASK).

This means that if L2 enables LBR and exits to L1, the LBRs will not be propagated from VMCB02 to VMCB01, because LBRV is disabled in VMCB01.

VMCB02 becomes the only source of truth for LBRs when L2 is running, regardless of LBRV being enabled by L1, drop svm_get_lbr_vmcb() and use svm->vmcb directly in its place.

Fixes 1d5a1b5860ed ("KVM: x86: nSVM: correctly virtualize LBR msrs when L2 is running")

Cc [email protected]

Signed-off-by Yosry Ahmed <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Signed-off-by Paolo Bonzini <[email protected]>

(cherry picked from commit 8a4821412cf2c1429fffa07c012dd150f2edf78c)

Signed-off-by Yosry Ahmed <[email protected]>

---
 arch/x86/kvm/svm/nested.c | 20 ++++++-----------
 arch/x86/kvm/svm/svm.c    | 47 +++++++++------------------------------
 2 files changed, 17 insertions(+), 50 deletions(-)

arch/x86/kvm/svm/nested.c +7 ~~-13~~

index e3f3e30fc89c..1577e6063da7 100644
@@ -600,11 +600,10 @@ static void nested_vmcb02_prepare_save(struct vcpu_svm *svm, struct vmcb *vmcb12
 		 */
 		svm_copy_lbrs(vmcb02, vmcb12);
 		vmcb02->save.dbgctl &= ~DEBUGCTL_RESERVED_BITS;
-		svm_update_lbrv(&svm->vcpu);
-
-	} else if (unlikely(vmcb01->control.virt_ext & LBR_CTL_ENABLE_MASK)) {
+	} else {
 		svm_copy_lbrs(vmcb02, vmcb01);
 	}
+	svm_update_lbrv(&svm->vcpu);
 }
 
 static inline bool is_evtinj_soft(u32 evtinj)

@@ -729,11 +728,7 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
 			svm->soft_int_next_rip = vmcb12_rip;
 	}
 
-	vmcb02->control.virt_ext            = vmcb01->control.virt_ext &
-					      LBR_CTL_ENABLE_MASK;
-	if (guest_can_use(vcpu, X86_FEATURE_LBRV))
-		vmcb02->control.virt_ext  |=
-			(svm->nested.ctl.virt_ext & LBR_CTL_ENABLE_MASK);
+	/* LBR_CTL_ENABLE_MASK is controlled by svm_update_lbrv() */
 
 	if (!nested_vmcb_needs_vls_intercept(svm))
 		vmcb02->control.virt_ext |= VIRTUAL_VMLOAD_VMSAVE_ENABLE_MASK;

@@ -1064,13 +1059,12 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
 		kvm_make_request(KVM_REQ_EVENT, &svm->vcpu);
 
 	if (unlikely(guest_can_use(vcpu, X86_FEATURE_LBRV) &&
-		     (svm->nested.ctl.virt_ext & LBR_CTL_ENABLE_MASK))) {
+		     (svm->nested.ctl.virt_ext & LBR_CTL_ENABLE_MASK)))
 		svm_copy_lbrs(vmcb12, vmcb02);
-		svm_update_lbrv(vcpu);
-	} else if (unlikely(vmcb01->control.virt_ext & LBR_CTL_ENABLE_MASK)) {
+	else
 		svm_copy_lbrs(vmcb01, vmcb02);
-		svm_update_lbrv(vcpu);
-	}
+
+	svm_update_lbrv(vcpu);
 
 	if (vnmi) {
 		if (vmcb02->control.int_ctl & V_NMI_BLOCKING_MASK)

arch/x86/kvm/svm/svm.c +10 ~~-37~~

88 lines not on this page · raw message

Signature · 1 line

2.52.0.158.g65b55ccf14-goog

Yosry Ahmed

2025-12-03 18:42 UTC #4 lore raw

PATCH 4/4 KVM: SVM: Fix redundant updates of LBR MSR intercepts

In reply to Yosry Ahmed

Don't update the LBR MSR intercept bitmaps if they're already up-to-date, as unconditionally updating the intercepts forces KVM to recalculate the MSR bitmaps for vmcb02 on every nested VMRUN. The redundant updates are functionally okay; however, they neuter an optimization in Hyper-V nested virtualization enlightenments and this manifests as a self-test failure.

In particular, Hyper-V lets L1 mark "nested enlightenments" as clean, i.e. tell KVM that no changes were made to the MSR bitmap since the last VMRUN. The hyperv_svm_test KVM selftest intentionally changes the MSR bitmap "without telling KVM about it" to verify that KVM honors the clean hint, correctly fails because KVM notices the changed bitmap anyway:

==== Test Assertion Failure ==== x86/hyperv_svm_test.c:120: vmcb->control.exit_code == 0x081 pid=193558 tid=193558 errno=4 - Interrupted system call

     1	0x0000000000411361: assert_on_unhandled_exception at processor.c:659
     2	0x0000000000406186: _vcpu_run at kvm_util.c:1699
     3	 (inlined by) vcpu_run at kvm_util.c:1710
     4	0x0000000000401f2a: main at hyperv_svm_test.c:175
     5	0x000000000041d0d3: __libc_start_call_main at libc-start.o:?
     6	0x000000000041f27c: __libc_start_main_impl at ??:?
     7	0x00000000004021a0: _start at ??:?

vmcb->control.exit_code == SVM_EXIT_VMMCALL

Do *not* fix this by skipping svm_hv_vmcb_dirty_nested_enlightenments() when svm_set_intercept_for_msr() performs a no-op change. changes to the L0 MSR interception bitmap are only triggered by full CPUID updates and MSR filter updates, both of which should be rare. Changing svm_set_intercept_for_msr() risks hiding unintended pessimizations like this one, and is actually more complex than this change.

Fixes fbe5e5f030c2 ("KVM: nSVM: Always recalculate LBR MSR intercepts in svm_update_lbrv()")

Cc [email protected]

Signed-off-by Yosry Ahmed <[email protected]>

Link https://patch.msgid.link/[email protected] open here

[Rewritten commit message based on mailing list discussion. - Paolo]

Reviewed-by Sean Christopherson <[email protected]>

Tested-by Sean Christopherson <[email protected]>

Signed-off-by Paolo Bonzini <[email protected]>

(cherry picked from commit 3fa05f96fc08dff5e846c2cc283a249c1bf029a1)

Signed-off-by Yosry Ahmed <[email protected]>

---
 arch/x86/kvm/svm/svm.c | 6 ++++++
 arch/x86/kvm/svm/svm.h | 1 +
 2 files changed, 7 insertions(+)

arch/x86/kvm/svm/svm.c +6 -0

index c5d66ded7ebb..097abdd1973b 100644
@@ -1014,6 +1014,9 @@ static void svm_recalc_lbr_msr_intercepts(struct kvm_vcpu *vcpu)
 	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;
+
 	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_LASTBRANCHFROMIP,
 			     !intercept, !intercept);
 	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_LASTBRANCHTOIP,

@@ -1026,6 +1029,8 @@ static void svm_recalc_lbr_msr_intercepts(struct kvm_vcpu *vcpu)
 	if (sev_es_guest(vcpu->kvm))
 		set_msr_interception(vcpu, svm->msrpm, MSR_IA32_DEBUGCTLMSR,
 				     !intercept, !intercept);
+
+	svm->lbr_msrs_intercepted = intercept;
 }
 
 static void __svm_enable_lbrv(struct kvm_vcpu *vcpu)

@@ -1470,6 +1475,7 @@ 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);

arch/x86/kvm/svm/svm.h +1 -0

index 81f00177f501..a73340a5c699 100644
@@ -288,6 +288,7 @@ 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;

Signature · 1 line

2.52.0.158.g65b55ccf14-goog