From: Yosry Ahmed Date: Wed, 12 Nov 2025 01:30:17 +0000 (+0000) Subject: KVM: SVM: Fix redundant updates of LBR MSR intercepts X-Git-Tag: v6.18-rc7~30^2 X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=3fa05f96fc08dff5e846c2cc283a249c1bf029a1;p=thirdparty%2Flinux.git KVM: SVM: Fix redundant updates of LBR MSR intercepts 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 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 Tested-by: Sean Christopherson Signed-off-by: Paolo Bonzini --- diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index 10c21e4c5406..9d29b2e7e855 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -705,7 +705,11 @@ void *svm_alloc_permissions_map(unsigned long size, gfp_t gfp_mask) static void svm_recalc_lbr_msr_intercepts(struct kvm_vcpu *vcpu) { - bool intercept = !(to_svm(vcpu)->vmcb->control.virt_ext & LBR_CTL_ENABLE_MASK); + 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; svm_set_intercept_for_msr(vcpu, MSR_IA32_LASTBRANCHFROMIP, MSR_TYPE_RW, intercept); svm_set_intercept_for_msr(vcpu, MSR_IA32_LASTBRANCHTOIP, MSR_TYPE_RW, intercept); @@ -714,6 +718,8 @@ static void svm_recalc_lbr_msr_intercepts(struct kvm_vcpu *vcpu) if (sev_es_guest(vcpu->kvm)) svm_set_intercept_for_msr(vcpu, MSR_IA32_DEBUGCTLMSR, MSR_TYPE_RW, intercept); + + svm->lbr_msrs_intercepted = intercept; } void svm_vcpu_free_msrpm(void *msrpm) @@ -1221,6 +1227,7 @@ 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 c856d8e0f95e..dd78e6402345 100644 --- a/arch/x86/kvm/svm/svm.h +++ b/arch/x86/kvm/svm/svm.h @@ -336,6 +336,7 @@ 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;