Re: [PATCH v7 08/12] KVM/x86/vPMU: Add APIs to support host save/restore the guest lbr stack

[Wei Wang]

On 07/09/2019 07:45 PM, Peter Zijlstra wrote:
> > +	 * config:        Actually this field won't be used by the perf core
> > +	 *                as this event doesn't have a perf counter.
> > +	 * sample_period: Same as above.
> If it's unused; why do we need to set it at all?

OK, we'll remove the unused fields.

> > +	 * sample_type:   tells the perf core that it is an lbr event.
> > +	 * branch_sample_type: tells the perf core that the lbr event works in
> > +	 *                the user callstack mode so that the lbr stack will be
> > +	 *                saved/restored on vCPU switching.
> Again; doesn't make sense. What does the user part have to do with
> save/restore? What happens when this vcpu thread drops to userspace for
> an assist?

This is a fake event which doesn't run the lbr on the host.
Returning to userspace doesn't need save/restore lbr, because userspace
wouldn't change those lbr msrs.

The event is created to save/restore lbr msrs on vcpu switching.
Host perf only do this save/restore for "user callstack mode" lbr event, 
so we
construct the event to be "user callstack mode" to reuse the host lbr 
save/restore.

An alternative method is to have KVM vcpu sched_in/out callback
save/restore those msrs, which don't need to create this fake event.

Please let me know if you prefer the second method.

Best,
Wei