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: <286AC319A985734F985F78AFA26841F73978F779@shsmsx102.ccr.corp.intel.com>
Date:   Fri, 7 Sep 2018 15:21:32 +0000
From:   "Wang, Wei W" <wei.w.wang@...el.com>
To:     Jann Horn <jannh@...gle.com>
CC:     kernel list <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        "Liang, Kan" <kan.liang@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Radim Kr?má? <rkrcmar@...hat.com>,
        "Xu, Like" <like.xu@...el.com>
Subject: RE: [PATCH v2 7/8] KVM: PMU: support to save/restore the guest lbr
 stack on vCPU switching

On Friday, September 7, 2018 10:37 PM, Jann Horn wrote:
> 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?

Thanks for reminding us this corner case. We didn’t consider that in this version. I think we could add guest_lbr_event_release() to kvm_arch_vcpu_destroy()

Best,
Wei