Re: [PATCH v11 08/11] KVM: x86/pmu: Emulate LBR feature via guest LBR event

Xu, Like
2020-05-19T12:24:46Z

On 2020/5/19 19:00, Peter Zijlstra wrote:
> On Thu, May 14, 2020 at 04:30:51PM +0800, Like Xu wrote:
>> +static inline bool event_is_oncpu(struct perf_event *event)
>> +{
>> + return event && event->oncpu != -1;
>> +}
>
>> +/*
>> + * It's safe to access LBR msrs from guest when they have not
>> + * been passthrough since the host would help restore or reset
>> + * the LBR msrs records when the guest LBR event is scheduled in.
>> + */
>> +static bool intel_pmu_access_lbr_msr(struct kvm_vcpu *vcpu,
>> + struct msr_data *msr_info, bool read)
>> +{
>> + struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
>> + u32 index = msr_info->index;
>> +
>> + if (!intel_is_valid_lbr_msr(vcpu, index))
>> + return false;
>> +
>> + if (!msr_info->host_initiated && !pmu->lbr_event)
>> + intel_pmu_create_lbr_event(vcpu);
>> +
>> + /*
>> + * Disable irq to ensure the LBR feature doesn't get reclaimed by the
>> + * host at the time the value is read from the msr, and this avoids the
>> + * host LBR value to be leaked to the guest. If LBR has been reclaimed,
>> + * return 0 on guest reads.
>> + */
>> + local_irq_disable();
>> + if (event_is_oncpu(pmu->lbr_event)) {
>> + if (read)
>> + rdmsrl(index, msr_info->data);
>> + else
>> + wrmsrl(index, msr_info->data);
>> + } else if (read)
>> + msr_info->data = 0;
>> + local_irq_enable();
>> +
>> + return true;
>> +}
> So this runs in the vCPU thread in host context to emulate the MSR
> access, right?
Yes, it's called to emulate MSR accesses when the guest LBR event is
scheduled on while the LBR stack MSRs have not been passthrough to the vCPU.

Thanks,
Like Xu
>