Re: [PATCH v1 01/11] perf/x86/core: Support KVM to assign a dedicated counter for guest PEBS

[Liang, Kan]

On 3/9/2020 6:04 AM, Peter Zijlstra wrote:
> On Fri, Mar 06, 2020 at 09:42:47AM -0500, Liang, Kan wrote:
> > On 3/6/2020 8:53 AM, Peter Zijlstra wrote:
> > > On Fri, Mar 06, 2020 at 01:56:55AM +0800, Luwei Kang wrote:
> > > > From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> > > >
> > > > The PEBS event created by host needs to be assigned specific counters
> > > > requested by the guest, which means the guest and host counter indexes
> > > > have to be the same or fail to create. This is needed because PEBS leaks
> > > > counter indexes into the guest. Otherwise, the guest driver will be
> > > > confused by the counter indexes in the status field of the PEBS record.
> > > >
> > > > A guest_dedicated_idx field is added to indicate the counter index
> > > > specifically requested by KVM. The dedicated event constraints would
> > > > constrain the counter in the host to the same numbered counter in guest.
> > > >
> > > > A intel_ctrl_guest_dedicated_mask field is added to indicate the enabled
> > > > counters for guest PEBS events. The IA32_PEBS_ENABLE MSR will be switched
> > > > during the VMX transitions if intel_ctrl_guest_owned is set.
> > >
> > > > +	/* the guest specified counter index of KVM owned event, e.g PEBS */
> > > > +	int				guest_dedicated_idx;
> > >
> > > We've always objected to guest 'owned' counters, they destroy scheduling
> > > freedom. Why are you expecting that to be any different this time?
> >
> > The new proposal tries to 'own' a counter by setting the event constraint.
> > It doesn't stop other events using the counter.
> > If there is high priority event which requires the same counter, scheduler
> > can still reject the request from KVM.
> > I don't think it destroys the scheduling freedom this time.
>
> Suppose your KVM thing claims counter 0/2 (ICL/SKL) for some random PEBS
> event, and then the host wants to use PREC_DIST.. Then one of them will
> be screwed for no reason what so ever.

The multiplexing should be triggered.

For host, if both user A and user B requires PREC_DIST, the multiplexing 
should be triggered for them.
Now, the user B is KVM. I don't think there is difference. The 
multiplexing should still be triggered. Why it is screwed?


> How is that not destroying scheduling freedom? Any other situation we'd
> have moved the !PREC_DIST PEBS event to another counter.

All counters are equivalent for them. It doesn't matter if we move it to 
another counter. There is no impact for the user.

In the new proposal, KVM user is treated the same as other host events 
with event constraint. The scheduler is free to choose whether or not to 
assign a counter for it.

Thanks,
Kan