linux-kernel - Re: [PATCH v1 1/8] perf/x86: add support to mask counters from host

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Date:   Mon, 5 Nov 2018 10:34:13 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        pbonzini@...hat.com, ak@...ux.intel.com, mingo@...hat.com,
        rkrcmar@...hat.com, like.xu@...el.com
Subject: Re: [PATCH v1 1/8] perf/x86: add support to mask counters from host

On Fri, Nov 02, 2018 at 05:08:31PM +0800, Wei Wang wrote:
> On 11/01/2018 10:52 PM, Peter Zijlstra wrote:
> > > @@ -723,6 +724,9 @@ static void perf_sched_init(struct perf_sched *sched, struct event_constraint **
> > >   	sched->max_weight	= wmax;
> > >   	sched->max_gp		= gpmax;
> > >   	sched->constraints	= constraints;
> > > +#ifdef CONFIG_CPU_SUP_INTEL
> > > +	sched->state.used[0]	= cpuc->intel_ctrl_guest_mask;
> > > +#endif
> > NAK.  This completely undermines the whole purpose of event scheduling.
> > 
> 
> Hi Peter,
> 
> Could you share more details how it would affect the host side event
> scheduling?

Not all counters are equal; suppose you have one of those chips that can
only do PEBS on counter 0, and then hand out 0 to the guest for some
silly event. That means nobody can use PEBS anymore.

> Would you have any suggestions?

I would suggest not to use virt in the first place of course ;-)

But whatever you do; you have to keep using host events to emulate the
guest PMU. That doesn't mean you can't improve things; that code is
quite insane from what you told earlier.