FAILED: patch "[PATCH] KVM: nSVM: Always recalculate LBR MSR intercepts in" failed to apply to 6.12-stable tree

FAILED: patch "[PATCH] KVM: nSVM: Always recalculate LBR MSR intercepts in" failed to apply to 6.12-stable tree

[gregkh]

The patch below does not apply to the 6.12-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 <stable@vger.kernel.org>.

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-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x fbe5e5f030c22ae717ee422aaab0e00ea84fab5e
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2025112046-confider-smelting-6296@gregkh' --subject-prefix 'PATCH 6.12.y' HEAD^..

Possible dependencies:



thanks,

greg k-h

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

From fbe5e5f030c22ae717ee422aaab0e00ea84fab5e Mon Sep 17 00:00:00 2001
From: Yosry Ahmed <yosry.ahmed@linux.dev>
Date: Sat, 8 Nov 2025 00:45:20 +0000
Subject: [PATCH] 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: stable@vger.kernel.org
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251108004524.1600006-3-yosry.ahmed@linux.dev
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 39538098002b..53201f13a43c 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -806,25 +806,29 @@ 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_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
@@ -853,13 +857,18 @@ void svm_update_lbrv(struct kvm_vcpu *vcpu)
 			    (is_guest_mode(vcpu) && guest_cpu_cap_has(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)

[PATCH 6.12.y 0/5] LBR virtualization fixes

[Yosry Ahmed]

This is a backport of LBR virtualization fixes that recently landed in
Linus's tree to 6.12.y.

Patch 1 is not a backport, it's introducing a helper that exists in
Linus's tree to make the following backports more straightforward to
apply.

Patch 2 should already be in queue-6.12, but it's included here as the
remaining patches depend on it.

Yosry Ahmed (5):
  KVM: SVM: Introduce svm_recalc_lbr_msr_intercepts()
  KVM: SVM: Mark VMCB_LBR dirty when MSR_IA32_DEBUGCTLMSR is updated
  KVM: nSVM: Always recalculate LBR MSR intercepts in svm_update_lbrv()
  KVM: nSVM: Fix and simplify LBR virtualization handling with nested
  KVM: SVM: Fix redundant updates of LBR MSR intercepts

 arch/x86/kvm/svm/nested.c | 20 +++-----
 arch/x86/kvm/svm/svm.c    | 96 ++++++++++++++++++++-------------------
 arch/x86/kvm/svm/svm.h    |  1 +
 3 files changed, 57 insertions(+), 60 deletions(-)

-- 
2.52.0.rc2.455.g230fcf2819-goog

[PATCH 6.12.y 1/5] KVM: SVM: Introduce svm_recalc_lbr_msr_intercepts()

[Yosry Ahmed]

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 <yosry.ahmed@linux.dev>
---
 arch/x86/kvm/svm/svm.c | 32 +++++++++++++++++++++-----------
 1 file changed, 21 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index c48a466db0c3..6deccc1b1669 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -995,18 +995,31 @@ 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))
@@ -1020,10 +1033,7 @@ 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
-- 
2.52.0.rc2.455.g230fcf2819-goog

[PATCH 6.12.y 2/5] KVM: SVM: Mark VMCB_LBR dirty when MSR_IA32_DEBUGCTLMSR is updated

[Yosry Ahmed]

The APM lists the DbgCtlMsr field as being tracked by the VMCB_LBR clean
bit.  Always clear the bit when MSR_IA32_DEBUGCTLMSR is updated.

The history is complicated, it was correctly cleared for L1 before
commit 1d5a1b5860ed ("KVM: x86: nSVM: correctly virtualize LBR msrs when
L2 is running").  At that point svm_set_msr() started to rely on
svm_update_lbrv() to clear the bit, but when nested virtualization
is enabled the latter does not always clear it even if MSR_IA32_DEBUGCTLMSR
changed. Go back to clearing it directly in svm_set_msr().

Fixes: 1d5a1b5860ed ("KVM: x86: nSVM: correctly virtualize LBR msrs when L2 is running")
Reported-by: Matteo Rizzo <matteorizzo@google.com>
Reported-by: evn@google.com
Co-developed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251108004524.1600006-2-yosry.ahmed@linux.dev
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit dc55b3c3f61246e483e50c85d8d5366f9567e188)
---
 arch/x86/kvm/svm/svm.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 6deccc1b1669..1e1210e5ef45 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3267,7 +3267,11 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 		if (data & DEBUGCTL_RESERVED_BITS)
 			return 1;
 
+		if (svm_get_lbr_vmcb(svm)->save.dbgctl == data)
+			break;
+
 		svm_get_lbr_vmcb(svm)->save.dbgctl = data;
+		vmcb_mark_dirty(svm->vmcb, VMCB_LBR);
 		svm_update_lbrv(vcpu);
 		break;
 	case MSR_VM_HSAVE_PA:
-- 
2.52.0.rc2.455.g230fcf2819-goog

[PATCH 6.12.y 3/5] KVM: nSVM: Always recalculate LBR MSR intercepts in svm_update_lbrv()

[Yosry Ahmed]

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: stable@vger.kernel.org
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251108004524.1600006-3-yosry.ahmed@linux.dev
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit fbe5e5f030c22ae717ee422aaab0e00ea84fab5e)

Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
---
 arch/x86/kvm/svm/svm.c | 29 +++++++++++++++++++----------
 1 file changed, 19 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 1e1210e5ef45..e3e570d11b26 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1014,26 +1014,30 @@ 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
@@ -1062,13 +1066,18 @@ 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)
-- 
2.52.0.rc2.455.g230fcf2819-goog

[PATCH 6.12.y 4/5] KVM: nSVM: Fix and simplify LBR virtualization handling with nested

[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).

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: stable@vger.kernel.org
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251108004524.1600006-4-yosry.ahmed@linux.dev
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 8a4821412cf2c1429fffa07c012dd150f2edf78c)

Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
---
 arch/x86/kvm/svm/nested.c | 20 ++++++-----------
 arch/x86/kvm/svm/svm.c    | 47 +++++++++------------------------------
 2 files changed, 17 insertions(+), 50 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 2dcb9c870d5a..ff6379fdf222 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -602,11 +602,10 @@ 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)
@@ -731,11 +730,7 @@ 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;
@@ -1066,13 +1061,12 @@ 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)
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index e3e570d11b26..a5e3dc482845 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1016,13 +1016,7 @@ static void svm_recalc_lbr_msr_intercepts(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;
-
-	/* 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);
+	to_svm(vcpu)->vmcb->control.virt_ext |= LBR_CTL_ENABLE_MASK;
 }
 
 void svm_enable_lbrv(struct kvm_vcpu *vcpu)
@@ -1033,36 +1027,15 @@ void svm_enable_lbrv(struct kvm_vcpu *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;
-
-	/*
-	 * Move the LBR msrs back to the vmcb01 to avoid copying them
-	 * on nested guest entries.
-	 */
-	if (is_guest_mode(vcpu))
-		svm_copy_lbrs(svm->vmcb01.ptr, svm->vmcb);
-}
-
-static struct vmcb *svm_get_lbr_vmcb(struct vcpu_svm *svm)
-{
-	/*
-	 * If LBR virtualization is disabled, the LBR MSRs are always kept in
-	 * vmcb01.  If LBR virtualization is enabled and L1 is running VMs of
-	 * its own, the MSRs are moved between vmcb01 and vmcb02 as needed.
-	 */
-	return svm->vmcb->control.virt_ext & LBR_CTL_ENABLE_MASK ? svm->vmcb :
-								   svm->vmcb01.ptr;
+	to_svm(vcpu)->vmcb->control.virt_ext &= ~LBR_CTL_ENABLE_MASK;
 }
 
 void svm_update_lbrv(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
 	bool current_enable_lbrv = svm->vmcb->control.virt_ext & LBR_CTL_ENABLE_MASK;
-	bool enable_lbrv = (svm_get_lbr_vmcb(svm)->save.dbgctl & DEBUGCTLMSR_LBR) ||
+	bool enable_lbrv = (svm->vmcb->save.dbgctl & DEBUGCTLMSR_LBR) ||
 			    (is_guest_mode(vcpu) && guest_can_use(vcpu, X86_FEATURE_LBRV) &&
 			    (svm->nested.ctl.virt_ext & LBR_CTL_ENABLE_MASK));
 
@@ -2991,19 +2964,19 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		msr_info->data = svm->tsc_aux;
 		break;
 	case MSR_IA32_DEBUGCTLMSR:
-		msr_info->data = svm_get_lbr_vmcb(svm)->save.dbgctl;
+		msr_info->data = svm->vmcb->save.dbgctl;
 		break;
 	case MSR_IA32_LASTBRANCHFROMIP:
-		msr_info->data = svm_get_lbr_vmcb(svm)->save.br_from;
+		msr_info->data = svm->vmcb->save.br_from;
 		break;
 	case MSR_IA32_LASTBRANCHTOIP:
-		msr_info->data = svm_get_lbr_vmcb(svm)->save.br_to;
+		msr_info->data = svm->vmcb->save.br_to;
 		break;
 	case MSR_IA32_LASTINTFROMIP:
-		msr_info->data = svm_get_lbr_vmcb(svm)->save.last_excp_from;
+		msr_info->data = svm->vmcb->save.last_excp_from;
 		break;
 	case MSR_IA32_LASTINTTOIP:
-		msr_info->data = svm_get_lbr_vmcb(svm)->save.last_excp_to;
+		msr_info->data = svm->vmcb->save.last_excp_to;
 		break;
 	case MSR_VM_HSAVE_PA:
 		msr_info->data = svm->nested.hsave_msr;
@@ -3276,10 +3249,10 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 		if (data & DEBUGCTL_RESERVED_BITS)
 			return 1;
 
-		if (svm_get_lbr_vmcb(svm)->save.dbgctl == data)
+		if (svm->vmcb->save.dbgctl == data)
 			break;
 
-		svm_get_lbr_vmcb(svm)->save.dbgctl = data;
+		svm->vmcb->save.dbgctl = data;
 		vmcb_mark_dirty(svm->vmcb, VMCB_LBR);
 		svm_update_lbrv(vcpu);
 		break;
-- 
2.52.0.rc2.455.g230fcf2819-goog

[PATCH 6.12.y 5/5] KVM: SVM: Fix redundant updates of LBR MSR intercepts

[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: stable@vger.kernel.org
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251112013017.1836863-1-yosry.ahmed@linux.dev
[Rewritten commit message based on mailing list discussion. - Paolo]
Reviewed-by: Sean Christopherson <seanjc@google.com>
Tested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 3fa05f96fc08dff5e846c2cc283a249c1bf029a1)

Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
---
 arch/x86/kvm/svm/svm.c | 6 ++++++
 arch/x86/kvm/svm/svm.h | 1 +
 2 files changed, 7 insertions(+)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index a5e3dc482845..71b32e64e801 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1000,6 +1000,9 @@ 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,
@@ -1012,6 +1015,8 @@ 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)
@@ -1450,6 +1455,7 @@ 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 1aa9b1e468cb..cada5db654b8 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -324,6 +324,7 @@ 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;
-- 
2.52.0.rc2.455.g230fcf2819-goog

Re: [PATCH 6.12.y 0/5] LBR virtualization fixes

[Greg KH]

On Thu, Nov 20, 2025 at 11:39:31PM +0000, Yosry Ahmed wrote:
> This is a backport of LBR virtualization fixes that recently landed in
> Linus's tree to 6.12.y.
> 
> Patch 1 is not a backport, it's introducing a helper that exists in
> Linus's tree to make the following backports more straightforward to
> apply.

Why not include the actual commit that adds that helper?

> Patch 2 should already be in queue-6.12, but it's included here as the
> remaining patches depend on it.

So this series will not apply?

confused,

greg k-h

Re: [PATCH 6.12.y 0/5] LBR virtualization fixes

[Yosry Ahmed]

On Fri, Nov 21, 2025 at 10:55:38AM +0100, Greg KH wrote:
> On Thu, Nov 20, 2025 at 11:39:31PM +0000, Yosry Ahmed wrote:
> > This is a backport of LBR virtualization fixes that recently landed in
> > Linus's tree to 6.12.y.
> > 
> > Patch 1 is not a backport, it's introducing a helper that exists in
> > Linus's tree to make the following backports more straightforward to
> > apply.
> 
> Why not include the actual commit that adds that helper?

It was introduced as part of a much larger commit,
160f143cc131 ("KVM: SVM: Manually recalc all MSR intercepts on userspace
MSR filter change"), which was a part of a 30+ patch series. The effort
and risk to backport all of that for the helper is not justifiable.

> 
> > Patch 2 should already be in queue-6.12, but it's included here as the
> > remaining patches depend on it.
> 
> So this series will not apply?

It applies on 6.12.y today, but if it gets merged to 6.12.y first [1],
then we may need to drop patch 2 from the to apply. There's a change
that git drops it though because it should be identical ot the patch
[1].

[1]https://lore.kernel.org/stable/20251121130147.807480816@linuxfoundation.org/

> 
> confused,
> 
> greg k-h