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: