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

[Peter Zijlstra <peterz@infradead.org>]

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?