linux-kernel - Re: [PATCH v7 08/12] KVM/x86/vPMU: Add APIs to support host save/restore the guest lbr stack

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Date:   Tue, 9 Jul 2019 13:45:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        pbonzini@...hat.com, ak@...ux.intel.com, kan.liang@...el.com,
        mingo@...hat.com, rkrcmar@...hat.com, like.xu@...el.com,
        jannh@...gle.com, arei.gonglei@...wei.com, jmattson@...gle.com
Subject: Re: [PATCH v7 08/12] KVM/x86/vPMU: Add APIs to support host
 save/restore the guest lbr stack

On Mon, Jul 08, 2019 at 09:23:15AM +0800, Wei Wang wrote:
> +int intel_pmu_enable_save_guest_lbr(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> +	struct perf_event *event;
> +
> +	/*
> +	 * The main purpose of this perf event is to have the host perf core
> +	 * help save/restore the guest lbr stack on vcpu switching. There is
> +	 * no perf counters allocated for the event.
> +	 *
> +	 * About the attr:
> +	 * exclude_guest: set to true to indicate that the event runs on the
> +	 *                host only.

That's a lie; it _never_ runs. You specifically don't want the host to
enable LBR so as not to corrupt the guest state.

> +	 * pinned:        set to false, so that the FLEXIBLE events will not
> +	 *                be rescheduled for this event which actually doesn't
> +	 *                need a perf counter.

Unparsable gibberish. Specifically by making it flexible it is
susceptible to rotation and there's no guarantee it will actually get
scheduled.

> +	 * config:        Actually this field won't be used by the perf core
> +	 *                as this event doesn't have a perf counter.
> +	 * sample_period: Same as above.

If it's unused; why do we need to set it at all?

> +	 * sample_type:   tells the perf core that it is an lbr event.
> +	 * branch_sample_type: tells the perf core that the lbr event works in
> +	 *                the user callstack mode so that the lbr stack will be
> +	 *                saved/restored on vCPU switching.

Again; doesn't make sense. What does the user part have to do with
save/restore? What happens when this vcpu thread drops to userspace for
an assist?

> +	 */
> +	struct perf_event_attr attr = {
> +		.type = PERF_TYPE_RAW,
> +		.size = sizeof(attr),
> +		.exclude_guest = true,
> +		.pinned = false,
> +		.config = 0,
> +		.sample_period = 0,
> +		.sample_type = PERF_SAMPLE_BRANCH_STACK,
> +		.branch_sample_type = PERF_SAMPLE_BRANCH_CALL_STACK |
> +				      PERF_SAMPLE_BRANCH_USER,
> +	};
> +
> +	if (pmu->vcpu_lbr_event)
> +		return 0;
> +
> +	event = perf_event_create(&attr, -1, current, NULL, NULL, false);
> +	if (IS_ERR(event)) {
> +		pr_err("%s: failed %ld\n", __func__, PTR_ERR(event));
> +		return -ENOENT;
> +	}
> +	pmu->vcpu_lbr_event = event;
> +
> +	return 0;
> +}