linux-kernel - Re: [PATCH v2 6/8] perf/x86/intel/lbr: guest requesting KVM for lbr stack save/restore

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Message-ID: <20180907141051.GP27886@tassilo.jf.intel.com>
Date:   Fri, 7 Sep 2018 07:10:51 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Wei Wang <wei.w.wang@...el.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>,
        "Liang, Kan" <kan.liang@...el.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "Xu, Like" <like.xu@...el.com>
Subject: Re: [PATCH v2 6/8] perf/x86/intel/lbr: guest requesting KVM for lbr
 stack save/restore

> This could achieve the above #1, but how would it solve #2 above? That is,
> after the guest uses the lbr feature for a while, the lbr stack has been
> passed through, then the guest doesn't use lbr any more, but the vCPU will
> still save/restore on switching?

If nothing accesses the MSR LBRs after a context switch in the guest
nothing gets saved/restored due to:

> > Also when the LBRs haven't been set to direct access the state doesn't
> > need to be saved.


-Andi