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 c5d66ded7ebb..097abdd1973b 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -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); diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h index 81f00177f501..a73340a5c699 100644 --- a/arch/x86/kvm/svm/svm.h +++ b/arch/x86/kvm/svm/svm.h @@ -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;