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SubjectRe: [RFC PATCH 2/2] KVM: SVM: Add MSR feature support for serializing LFENCE
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On 2/13/2018 10:22 AM, Paolo Bonzini wrote:
> On 08/02/2018 23:58, Tom Lendacky wrote:
>> Create an entry in the new MSR as a feature framework to allow a guest to
>> recognize LFENCE as a serializing instruction on AMD processors. The MSR
>> can only be set by the host, any write by the guest will be ignored. A
>> read by the guest will return the value as set by the host. In this way,
>> the support to expose the feature to the guest is controlled by the
>> hypervisor.
>>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>> ---
>> arch/x86/kvm/svm.c | 16 ++++++++++++++++
>> arch/x86/kvm/x86.c | 6 ++++++
>> 2 files changed, 22 insertions(+)
>>
>> @@ -4047,6 +4052,17 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>> case MSR_VM_IGNNE:
>> vcpu_unimpl(vcpu, "unimplemented wrmsr: 0x%x data 0x%llx\n", ecx, data);
>> break;
>> + case MSR_F10H_DECFG:
>> + /* Only the host can set this MSR, silently ignore */
>> + if (!msr->host_initiated)
>> + break;
>
> Just one thing I'm wondering, should we #GP if the guest attempts to
> clear MSR_F10H_DECFG_LFENCE_SERIALIZE?

It would be more consistent with other entries to do "return 1" here
instead. The current kernel code that writes this bit is using
msr_set_bit(), so a #GP is caught and handled.

Thanks,
Tom

>
> Thanks,
>
> Paolo
>
>> +
>> + /* Check the supported bits */
>> + if (!kvm_valid_msr_feature(MSR_F10H_DECFG, data))
>> + return 1;
>> +
>> + svm->msr_decfg = data;
>> + break;
>> case MSR_IA32_APICBASE:
>> if (kvm_vcpu_apicv_active(vcpu))
>> avic_update_vapic_bar(to_svm(vcpu), data);
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 4251c34..21ec73b 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -1060,7 +1060,13 @@ struct kvm_msr_based_features {
>> u64 value; /* MSR value */
>> };
>>
>> +static const struct x86_cpu_id msr_decfg_match[] = {
>> + { X86_VENDOR_AMD, X86_FAMILY_ANY, X86_MODEL_ANY, X86_FEATURE_LFENCE_RDTSC },
>> + {}
>> +};
>> +
>> static struct kvm_msr_based_features msr_based_features[] = {
>> + { MSR_F10H_DECFG, MSR_F10H_DECFG_LFENCE_SERIALIZE, msr_decfg_match },
>> {}
>> };
>>
>>
>

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