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

From: Peter Zijlstra
Date: Mon Jul 08 2019 - 10:53:47 EST

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.