Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750862AbcKFUx5 (ORCPT ); Sun, 6 Nov 2016 15:53:57 -0500 Received: from mail-vk0-f66.google.com ([209.85.213.66]:34725 "EHLO mail-vk0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750720AbcKFUxz (ORCPT ); Sun, 6 Nov 2016 15:53:55 -0500 MIME-Version: 1.0 In-Reply-To: References: <20161101013749.4003-1-khuey@kylehuey.com> <20161101013749.4003-8-khuey@kylehuey.com> From: Kyle Huey Date: Sun, 6 Nov 2016 12:53:53 -0800 Message-ID: Subject: Re: [PATCH v8 7/7] KVM: x86: virtualize cpuid faulting To: David Matlack Cc: Paolo Bonzini , "Robert O'Callahan" , Thomas Gleixner , Andy Lutomirski , Ingo Molnar , "H. Peter Anvin" , X86 ML , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Jeff Dike , Richard Weinberger , Alexander Viro , Shuah Khan , Dave Hansen , Borislav Petkov , Peter Zijlstra , Boris Ostrovsky , Len Brown , "Rafael J. Wysocki" , Dmitry Safonov , "linux-kernel@vger.kernel.org" , Eduardo Habkost Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3392 Lines: 86 On Fri, Nov 4, 2016 at 5:09 PM, David Matlack wrote: > On Fri, Nov 4, 2016 at 2:57 PM, Paolo Bonzini wrote: >> >> On 04/11/2016 21:34, David Matlack wrote: >>> On Mon, Oct 31, 2016 at 6:37 PM, Kyle Huey wrote: >>>> + case MSR_PLATFORM_INFO: >>>> + /* cpuid faulting is supported */ >>>> + msr_info->data = PLATINFO_CPUID_FAULT; >>>> + break; >>> >>> This could break save/restore, if for example, a VM is migrated to a >>> version of KVM without MSR_PLATFORM_INFO support. I think the way to >>> handle this is to make MSR_PLATFORM_INFO writeable (but only from >>> userspace) so that hypervisors can defend themselves (by setting this >>> MSR to 0). >> >> Right---and with my QEMU hat on, this feature will have to be enabled >> manually on the command line because of the way QEMU supports running >> with old kernels. :( This however does not impact the KVM patch. >> >> We may decide that, because CPUID faulting doesn't have a CPUID bit and >> is relatively a "fringe" feature, we are okay if the kernel enables this >> unconditionally and then userspace can arrange to block migration (in >> QEMU this would use a subsection). David, Eduardo, opinions? > > Sounds reasonable. Accurate CPU virtualization might be another reason > to disable this feature from userspace. > > My worry is a kernel rollback, where migrating to an older kernel > version is unavoidable. > >> >>> >>>> + case MSR_MISC_FEATURES_ENABLES: >>>> + msr_info->data = 0; >>>> + if (vcpu->arch.cpuid_fault) >>>> + msr_info->data |= CPUID_FAULT_ENABLE; >>>> + break; >>> >>> MSR_MISC_FEATURES_ENABLES should be added to emulated_msrs[] so that >>> the hypervisor will maintain the value of CPUID_FAULT_ENABLE across a >>> save/restore. >> >> This is definitely necessary. Thanks David. >> >> Paolo >> >>>> default: >>>> if (kvm_pmu_is_valid_msr(vcpu, msr_info->index)) >>>> return kvm_pmu_get_msr(vcpu, msr_info->index, &msr_info->data); >>>> if (!ignore_msrs) { >>>> vcpu_unimpl(vcpu, "unhandled rdmsr: 0x%x\n", msr_info->index); >>>> return 1; >>>> } else { >>>> vcpu_unimpl(vcpu, "ignored rdmsr: 0x%x\n", msr_info->index); >>>> @@ -7493,16 +7507,18 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event) >>>> kvm_update_dr0123(vcpu); >>>> vcpu->arch.dr6 = DR6_INIT; >>>> kvm_update_dr6(vcpu); >>>> vcpu->arch.dr7 = DR7_FIXED_1; >>>> kvm_update_dr7(vcpu); >>>> >>>> vcpu->arch.cr2 = 0; >>>> >>>> + vcpu->arch.cpuid_fault = false; >>>> + >>>> kvm_make_request(KVM_REQ_EVENT, vcpu); >>>> vcpu->arch.apf.msr_val = 0; >>>> vcpu->arch.st.msr_val = 0; >>>> >>>> kvmclock_reset(vcpu); >>>> >>>> kvm_clear_async_pf_completion_queue(vcpu); >>>> kvm_async_pf_hash_reset(vcpu); >>>> -- >>>> 2.10.2 >>>> >>>> -- >>>> To unsubscribe from this list: send the line "unsubscribe kvm" in >>>> the body of a message to majordomo@vger.kernel.org >>>> More majordomo info at http://vger.kernel.org/majordomo-info.html >>> Thanks for the feedback. A new patch series is coming up. - Kyle