KVM: SVM: Mark VMCB_LBR dirty when MSR_IA32_DEBUGCTLMSR is updated

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>

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 76055c0ba177e49ae14d1fa0ef2a23b56e473276..39538098002b95643a31c33e3d1bbd2debd386f5 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3004,7 +3004,11 @@ 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: