linux-kernel - Re: [PATCH v2 7/8] KVM: PMU: support to save/restore the guest lbr stack on vCPU switching

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Message-ID: <CAG48ez1MX18nNgq1mih4dBmX5C_-0ZoCAayzhuzyaDvxYmJrdw@mail.gmail.com>
Date:   Fri, 7 Sep 2018 16:36:54 +0200
From:   Jann Horn <jannh@...gle.com>
To:     wei.w.wang@...el.com
Cc:     kernel list <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>, kan.liang@...el.com,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        like.xu@...el.com
Subject: Re: [PATCH v2 7/8] KVM: PMU: support to save/restore the guest lbr
 stack on vCPU switching

On Fri, Sep 7, 2018 at 4:28 PM Wei Wang <wei.w.wang@...el.com> wrote:
> This patch adds support to KVM to save/restore the lbr stack on vCPU
> context switching.
>
> When the guest sets the ACTIVE bit of MSR_KVM_PV_LBR_CTRL, a perf event
> is created on the host for the related vCPU. This perf event ensures the
> LBR stack to be saved/restored when the vCPU thread is scheduled out/in.
> The perf event is removed and freed when the guest clears the ACTIVE
> bit.
[...]
> +void guest_lbr_event_release(struct kvm_pmu *pmu)
> +{
> +       struct perf_event *event = pmu->guest_lbr_event;
> +
> +       if (unlikely(!pmu->guest_lbr_event)) {
> +               pr_err("%s: guest_lbr_event already freed\n", __func__);
> +               return;
> +       }
> +
> +       if (event) {
> +               event->pmu->stop(event, PERF_EF_UPDATE);
> +               perf_event_release_kernel(event);
> +       }
> +       pmu->guest_lbr_event = NULL;
> +}

Is there some guarantee that this method will be called when the vCPU
is torn down on guest exit?