Re: [PATCH v3 04/17] perf: x86/ds: Handle guest PEBS overflow PMI and inject it to guest

Peter Zijlstra
2021-01-15T12:03:16Z

On Thu, Jan 14, 2021 at 11:39:00AM +0800, Xu, Like wrote:

> > Why do we need to? Can't we simply always forward the PMI if the guest
> > has bits set in MSR_IA32_PEBS_ENABLE ? Surely we can access the guest
> > MSRs at a reasonable rate..
> >
> > Sure, it'll send too many PMIs, but is that really a problem?
>
> More vPMI means more guest irq handler calls and
> more PMI virtualization overhead.

Only if you have both guest and host PEBS. And in that case I really
can't be arsed about some overhead to the guest.

> In addition,
> the correctness of some workloads (RR?) depends on
> the correct number of PMIs and the PMI trigger times
> and virt may not want to break this assumption.

Are you sure? Spurious NMI/PMIs are known to happen anyway. We have far
too much code to deal with them.

> > > + * If PEBS interrupt threshold on host is not exceeded in a NMI, there
> > > + * must be a PEBS overflow PMI generated from the guest PEBS counters.
> > > + * There is no ambiguity since the reported event in the PMI is guest
> > > + * only. It gets handled correctly on a case by case base for each event.
> > > + *
> > > + * Note: KVM disables the co-existence of guest PEBS and host PEBS.
> > Where; I need a code reference here.
>
> How about:
>
> Note: KVM will disable the co-existence of guest PEBS and host PEBS.
> In the intel_guest_get_msrs(), when we have host PEBS ctrl bit(s) enabled,
> KVM will clear the guest PEBS ctrl enable bit(s) before vm-entry.
> The guest PEBS users should be notified of this runtime restriction.

Since you had me look at that function, can clean up that
CONFIG_RETPOLINE crud and replace it with static_call() ?