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: <20180918025616.GA10360@tassilo.jf.intel.com>
Date:   Mon, 17 Sep 2018 19:56:16 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     "Gonglei (Arei)" <arei.gonglei@...wei.com>
Cc:     Wei Wang <wei.w.wang@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "kan.liang@...el.com" <kan.liang@...el.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "like.xu@...el.com" <like.xu@...el.com>
Subject: Re: [PATCH v2 7/8] KVM: PMU: support to save/restore the guest lbr
 stack on vCPU switching

> > From: Like Xu <like.xu@...el.com>
> > 
> > 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.
> > 
> 
> What about live migration? Does LBR stack need to be saved on the source side and
> restored on the dest side with the passthrough mode?

Yes it should. Either for call stack LBR, or when it is frozen/disabled.

When it's not frozen/disabled and not in call stack LBR mode it likely doesn't
hurt either, but it's not strictly needed because it will
be replaced so quickly.

-Andi