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

[Andi Kleen]

> > 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.

-Andi