Re: [PATCH v2 7/8] KVM: PMU: support to save/restore the guest lbr stack on vCPU switching

[Wei Wang]

On 09/18/2018 10:56 AM, Andi Kleen wrote:
> > > From: Like Xu <like.xu@xxxxxxxxx>
> > >
> > > This patch adds support to KVM to save/restore the lbr stack on vCPU
> > > context switching.
> > >
> > > When the guest sets the ACTIVE bit of MSR_KVM_PV_LBR_CTRL, a perf event
> > > is created on the host for the related vCPU. This perf event ensures the
> > > LBR stack to be saved/restored when the vCPU thread is scheduled out/in.
> > > The perf event is removed and freed when the guest clears the ACTIVE
> > > bit.
> > What about live migration? Does LBR stack need to be saved on the source side and
> > restored on the dest side with the passthrough mode?
> Yes it should. Either for call stack LBR, or when it is frozen/disabled.
>
> When it's not frozen/disabled and not in call stack LBR mode it likely doesn't
> hurt either, but it's not strictly needed because it will
> be replaced so quickly.

Yes, should be supported. We are working on v3 with the suggested lazy 
save approach, and will send it out shortly.

Best,
Wei