[PATCH 6.1.y] KVM: SVM: Mark VMCB_LBR dirty when MSR_IA32_DEBUGCTLMSR is updated

[Sasha Levin <sashal@kernel.org>]

From: Yosry Ahmed <yosry.ahmed@linux.dev>

[ Upstream commit dc55b3c3f61246e483e50c85d8d5366f9567e188 ]

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>
[ Open coded svm_get_lbr_vmcb() call ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/x86/kvm/svm/svm.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index f56dcbbbf7341..00fb1c18e23a8 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3053,11 +3053,17 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 		if (data & DEBUGCTL_RESERVED_BITS)
 			return 1;
 
-		if (svm->vmcb->control.virt_ext & LBR_CTL_ENABLE_MASK)
+		if (svm->vmcb->control.virt_ext & LBR_CTL_ENABLE_MASK) {
+			if (svm->vmcb->save.dbgctl == data)
+				break;
 			svm->vmcb->save.dbgctl = data;
-		else
+		} else {
+			if (svm->vmcb01.ptr->save.dbgctl == data)
+				break;
 			svm->vmcb01.ptr->save.dbgctl = data;
+		}
 
+		vmcb_mark_dirty(svm->vmcb, VMCB_LBR);
 		svm_update_lbrv(vcpu);
 
 		break;
-- 
2.51.0