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

From: Wei Wang
Date: Tue Sep 18 2018 - 05:53:06 EST

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

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.