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

[Xu, Like]

On 2021/1/15 20:01, Peter Zijlstra wrote:
> 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.

Less overhead makes everyone happier.

Ah, can I assume that you're fine with disabling the
co-existence of guest PEBS and host PEBS as the first upstream step ?

> > 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.

https://lore.kernel.org/lkml/20170628130748.GI5981@leverpostej/T/

In the rr workload, the commit change "the PMI interrupts in skid region 
should be dropped"
is reverted since some users complain that:

> It seems to me that it might be reasonable to ignore the interrupt if
> the purpose of the interrupt is to trigger sampling of the CPUs
> register state.  But if the interrupt will trigger some other
> operation, such as a signal on an fd, then there's no reason to drop
> it.

I assume that if the PMI drop is unacceptable, either will spurious PMI 
injection.

I'm pretty open if you insist that we really need to do this for guest PEBS 
enabling.

> > > > +	 * 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() ?

Sure. Let me try it.

---
thx, likexu