linux-kernel - RE: [PATCH v3 4/5] KVM/x86/vPMU: Add APIs to support host save/restore the guest lbr stack

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Message-ID: <286AC319A985734F985F78AFA26841F7397FD362@shsmsx102.ccr.corp.intel.com>
Date:   Thu, 27 Sep 2018 13:45:31 +0000
From:   "Wang, Wei W" <wei.w.wang@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
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>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Liang, Kan" <kan.liang@...el.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "Xu, Like" <like.xu@...el.com>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "arei.gonglei@...wei.com" <arei.gonglei@...wei.com>
Subject: RE: [PATCH v3 4/5] KVM/x86/vPMU: Add APIs to support host
 save/restore the guest lbr stack

On Thursday, September 20, 2018 8:32 PM, Peter Zijlstra wrote:
> On Thu, Sep 20, 2018 at 06:05:58PM +0800, Wei Wang wrote:
> How do you deal with that situation, where the LBR is in use by a host event?

I guess you were referring to this use case: "perf record -b qemu-system-x86_64..", where the qemu process also needs lbr.

I also discussed with Andi about this in v1, where we switch the lbr state on VMX transitions. That could achieve the above, but adds too much overhead to VMX transitions, which are frequent. Considering the above usage is not common (it's more common to just have the task in the guest use the lbr feature), we decided to not take that usage into account, and switch lbr state only on the vCPU switching.

Best,
Wei