linux-kernel - Re: [PATCH v7 10/12] KVM/x86/lbr: lazy save the guest lbr stack

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Date:   Mon, 8 Jul 2019 16:53:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        pbonzini@...hat.com, ak@...ux.intel.com, kan.liang@...el.com,
        mingo@...hat.com, rkrcmar@...hat.com, like.xu@...el.com,
        jannh@...gle.com, arei.gonglei@...wei.com, jmattson@...gle.com
Subject: Re: [PATCH v7 10/12] KVM/x86/lbr: lazy save the guest lbr stack

On Mon, Jul 08, 2019 at 09:23:17AM +0800, Wei Wang wrote:
> When the vCPU is scheduled in:
> - if the lbr feature was used in the last vCPU time slice, set the lbr
>   stack to be interceptible, so that the host can capture whether the
>   lbr feature will be used in this time slice;
> - if the lbr feature wasn't used in the last vCPU time slice, disable
>   the vCPU support of the guest lbr switching.
> 
> Upon the first access to one of the lbr related MSRs (since the vCPU was
> scheduled in):
> - record that the guest has used the lbr;
> - create a host perf event to help save/restore the guest lbr stack;
> - pass the stack through to the guest.

I don't understand a word of that.

Who cares if the LBR MSRs are touched; the guest expects up-to-date
values when it does reads them.