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Date:   Fri, 7 Sep 2018 07:10:51 -0700
From:   Andi Kleen <ak@linux.intel.com>
To:     Wei Wang <wei.w.wang@intel.com>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "pbonzini@redhat.com" <pbonzini@redhat.com>,
        "Liang, Kan" <kan.liang@intel.com>,
        "peterz@infradead.org" <peterz@infradead.org>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "rkrcmar@redhat.com" <rkrcmar@redhat.com>,
        "Xu, Like" <like.xu@intel.com>
Subject: Re: [PATCH v2 6/8] perf/x86/intel/lbr: guest requesting KVM for lbr
 stack save/restore
Message-ID: <20180907141051.GP27886@tassilo.jf.intel.com>
References: <1536233456-12173-1-git-send-email-wei.w.wang@intel.com>
 <1536233456-12173-7-git-send-email-wei.w.wang@intel.com>
 <20180907032707.GN27886@tassilo.jf.intel.com>
 <5B920B96.6080803@intel.com>
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> 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