linux-kernel - Re: [PATCH v1 4/4] KVM/vmx: enable lbr for the guest

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Date:   Wed, 27 Sep 2017 09:27:46 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     Andi Kleen <ak@...ux.intel.com>
CC:     virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, pbonzini@...hat.com, mst@...hat.com,
        rkrcmar@...hat.com, mingo@...hat.com
Subject: Re: [PATCH v1 4/4] KVM/vmx: enable lbr for the guest

On 09/27/2017 12:41 AM, Andi Kleen wrote:
>> 1) vCPU context switching and guest side task switching are not identical.
>> That is, when the vCPU is scheduled out, the guest task on the vCPU may not
> guest task lifetime has nothing to do with this. It's completely independent
> of what you do here on the VCPU level.
>
>> run out its time slice yet, so the task will continue to run when the vCPU
>> is
>> scheduled in by the host (lbr wasn't save by the guest task when the vCPU is
>> scheduled out in this case).
>>
>> It is possible to have the vCPU which runs the guest task (in use of lbr)
>> scheduled
>> out, followed by a new host task being scheduled in on the pCPU to run.
>> It is not guaranteed that the new host task does not use the LBR feature on
>> the
>> pCPU.
> Sure it may use the LBR, and the normal perf context switch
> will switch it and everything works fine.
>
> It's like any other per-task LBR user.

OK, I see the point, thanks.

Why couldn't we save the LBR_SELECT via task switching too?


Best,
Wei