Re: [PATCH v8 7/7] KVM: x86: virtualize cpuid faulting

[Paolo Bonzini]

On 04/11/2016 21:34, David Matlack wrote:
> On Mon, Oct 31, 2016 at 6:37 PM, Kyle Huey <me@xxxxxxxxxxxx> 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?

> 
>> +       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
>>
>> --
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>