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: <5BA0CC1E.6030202@intel.com>
Date:   Tue, 18 Sep 2018 17:57:50 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     Andi Kleen <ak@...ux.intel.com>,
        "Gonglei (Arei)" <arei.gonglei@...wei.com>
CC:     "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

On 09/18/2018 10:56 AM, Andi Kleen wrote:
>>> 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.

Yes, should be supported. We are working on v3 with the suggested lazy 
save approach, and will send it out shortly.

Best,
Wei