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SubjectRe: [RFC PATCH 2/2] KVM: SVM: Add MSR feature support for serializing LFENCE
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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?

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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