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>>>> >>>> - if (svm_get_feature_msr(ecx, &supported_de_cfg)) >>>> + if (WARN_ON_ONCE(svm_get_feature_msr(ecx, &supported_de_cfg))) >>>> return 1; >>>> >>>> - if (data & ~supported_de_cfg) >>>> + if (data & ~supported_de_cfg) { >>>> + pr_warn("DE_CFG supported = %llx, WRMSR = %llx\n", >>>> + supported_de_cfg, data); >>>> return 1; >>>> + } >>>> >>>> /* >>>> * Don't let the guest change the host-programmed value. The >>>> @@ -3207,8 +3210,11 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr) >>>> * are completely unknown to KVM, and the one bit known to KVM >>>> * is simply a reflection of hardware capabilities. >>>> */ >>>> - if (!msr->host_initiated && data != svm->msr_decfg) >>>> + if (!msr->host_initiated && data != svm->msr_decfg) { >>>> + pr_warn("DE_CFG current = %llx, WRMSR = %llx\n", >>>> + svm->msr_decfg, data); >>>> return 1; >>>> + } >>>> >>>> svm->msr_decfg = data; >>>> break; >>>> >>>> base-commit: fac04efc5c793dccbd07e2d59af9f90b7fc0dca4 >>> >>> Relevant dmesg output with some context below. VM locked up as expected. >>> >>> [ 85.834971] vfio-pci 0000:0c:00.0: resetting >>> [ 85.937573] vfio-pci 0000:0c:00.0: reset done >>> [ 86.494210] vfio-pci 0000:0c:00.0: resetting >>> [ 86.494264] vfio-pci 0000:0c:00.1: resetting >>> [ 86.761442] vfio-pci 0000:0c:00.0: reset done >>> [ 86.761480] vfio-pci 0000:0c:00.1: reset done >>> [ 86.762392] vfio-pci 0000:0c:00.0: resetting >>> [ 86.865462] vfio-pci 0000:0c:00.0: reset done >>> [ 86.977360] virbr0: port 1(vnet1) entered learning state >>> [ 88.993052] virbr0: port 1(vnet1) entered forwarding state >>> [ 88.993057] virbr0: topology change detected, propagating >>> [ 103.459114] kvm_amd: DE_CFG current = 0, WRMSR = 2 >>> [ 161.442032] virbr0: port 1(vnet1) entered disabled state // VM shut down >> >> That is the MSR_AMD64_DE_CFG_LFENCE_SERIALIZE bit. Yeah, that actually >> does change the behavior of LFENCE and isn't just a reflection of the >> hardware. >> >> Linux does set that bit on boot, too (if LFENCE always serializing isn't >> advertised 8000_0021_EAX[2]), so I'm kind of surprised it didn't pop up >> there. > > Linux may be running afoul of this, but it would only become visible if someone > checked dmesg. Even the "unsafe" MSR accesses in Linux gracefully handle faults > these days, the only symptom would be a WARN. > >> I imagine that the above CPUID bit isn't set, so an attempt is made to >> set the MSR bit. > > Yep. And LFENCE_RDTSC _is_ supported, otherwise the supported_de_cfg check would > have failed. Which means it's a-ok for the guest to set the bit, i.e. KVM won't > let the guest incorrectly think it's running on CPU for which LFENCE is serializing. > > Unless you (Tom) disagree, I vote to simply drop the offending code, i.e. make > all supported bits fully writable from the guest. KVM is firmly in the wrong here, > and I can't think of any reason to disallow the guest from clearing LFENCE_SERIALIZE. > > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c > index 6a350cee2f6c..5a82ead3bf0f 100644 > --- a/arch/x86/kvm/svm/svm.c > +++ b/arch/x86/kvm/svm/svm.c > @@ -3201,15 +3201,6 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr) > if (data & ~supported_de_cfg) > return 1; > > - /* > - * Don't let the guest change the host-programmed value. The > - * MSR is very model specific, i.e. contains multiple bits that > - * are completely unknown to KVM, and the one bit known to KVM > - * is simply a reflection of hardware capabilities. > - */ > - if (!msr->host_initiated && data != svm->msr_decfg) > - return 1; > - That works for me. Thanks, Tom > svm->msr_decfg = data; > break; > } >