Re: [PATCH v2 6/8] perf/x86/intel/lbr: guest requesting KVM for lbr stack save/restore

[Andi Kleen]

> How would you realize the function of saving/restoring the lbr stack on the host?
> 
> Here, we create a perf event on the host (please see guest_lbr_event_create on patch 7), which essentially satisfies all the conditions (e.g. increases cpuc->lbr_users) that are required to have the lbr stack saved/restored on the vCPU switching.
> 
> If we want to stop the host side lbr stack save/restore for the vCPU, we need accordingly to call guest_lbr_event_release (in patch 7) to destroy that perf event (the host doesn't automatically stop saving the lbr stack for the vCPU if that perf event is still there).
> 
> When would you call that release function? (we all know that the lbr doesn't need to be saved when the guest is not using it, but we need to destroy that perf event to achieve "doesn't need to be saved")

Maybe set a timer on DEBUGCTL LBR=0 ? A timer would provide hysteresis, so that quick toggles
(like in a PMI handler) wouldn't do anything expensive.

It needs new interfaces for perf anyways because we need to access the LBR state from
the last save.

-Andi