Messages in this thread | | | Subject | Re: [PATCH v14 8/9] KVM: x86: virtualize cpuid faulting | From | Andy Lutomirski <> | Date | Fri, 27 Jul 2018 13:46:06 -0700 |
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> On Jul 27, 2018, at 1:28 PM, Jim Mattson <jmattson@google.com> wrote: > >> On Fri, Jul 27, 2018 at 12:41 PM, Andy Lutomirski <luto@kernel.org> wrote: >>> On Wed, Feb 8, 2017 at 12:09 AM, Kyle Huey <me@kylehuey.com> wrote: >>> Hardware support for faulting on the cpuid instruction is not required to >>> emulate it, because cpuid triggers a VM exit anyways. KVM handles the relevant >>> MSRs (MSR_PLATFORM_INFO and MSR_MISC_FEATURES_ENABLE) and upon a >>> cpuid-induced VM exit checks the cpuid faulting state and the CPL. >>> kvm_require_cpl is even kind enough to inject the GP fault for us. >>> >>> Signed-off-by: Kyle Huey <khuey@kylehuey.com> >>> Reviewed-by: David Matlack <dmatlack@google.com> >>> --- >>> ... >>> @@ -7613,16 +7636,19 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event) >>> >>> kvm_clear_async_pf_completion_queue(vcpu); >>> kvm_async_pf_hash_reset(vcpu); >>> vcpu->arch.apf.halted = false; >>> >>> if (!init_event) { >>> kvm_pmu_reset(vcpu); >>> vcpu->arch.smbase = 0x30000; >>> + >>> + vcpu->arch.msr_platform_info = MSR_PLATFORM_INFO_CPUID_FAULT; >>> + vcpu->arch.msr_misc_features_enables = 0; >> >> Jim, I assume you're worried about this bit? It seems like >> msr_platform_info should maybe be initialized to zero to avoid causing >> an unintended migration issue. > > Initializing this bit to zero helps with migration, but then if the > CPUID faulting bits in both MSRs are set, userspace has to take pains > to ensure that MSR_PLATFORM_INFO is restored first, or the > MSR_MISC_FEATURES_ENABLES value will be rejected.
The code could drop the constraint and just let the entry possibly fail if the MSRs are set wrong
> > I'm also concerned about the 0 in the "Maximum Non-Turbo Ratio" field > feeding into someone's calculated TSC frequency.
Hmm. I don’t have a good answer to that. Are there any real CPUs that have this MSR but don’t have that field?
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