Re: [RFC] perf/x86: Only expose userspace rdpmc for events on current CPU

[Peter Zijlstra]

On Tue, Jan 12, 2021 at 10:16:50AM -0600, Rob Herring wrote:
> On Tue, Jan 12, 2021 at 9:33 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, Jan 07, 2021 at 05:01:36PM -0700, Rob Herring wrote:
> > > Userspace access using rdpmc only makes sense if the event is valid for
> > > the current CPU. However, cap_user_rdpmc is currently set no matter which
> > > CPU the event is associated with. The result is userspace reading another
> > > CPU's event thinks it can use rdpmc to read the counter. In doing so, the
> > > wrong counter will be read.
> >
> > Don't do that then?
> 
> I could check this in userspace I suppose, but then it's yet another
> thing the rdpmc loop has to check. I think it's better to not add more
> overhead there.

So all this was designed for self monitoring; attempting rdpmc on an
event not for yourself is out of spec.

> > > diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> > > index a88c94d65693..6e6d4c1d03ca 100644
> > > --- a/arch/x86/events/core.c
> > > +++ b/arch/x86/events/core.c
> > > @@ -2490,7 +2490,8 @@ void arch_perf_update_userpage(struct perf_event *event,
> > >       userpg->cap_user_time = 0;
> > >       userpg->cap_user_time_zero = 0;
> > >       userpg->cap_user_rdpmc =
> > > -             !!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED);
> > > +             !!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED) &&
> > > +             (event->oncpu == smp_processor_id());
> > >       userpg->pmc_width = x86_pmu.cntval_bits;
> > >
> > >       if (!using_native_sched_clock() || !sched_clock_stable())
> >
> > Isn't that a nop? That is, from the few sites I checked, we're always
> > calling this on the event's CPU.
> 
> If cpu0 opens and mmaps an event for cpu1, then cpu0 will see
> cap_user_rdpmc set and think it can use rdpmc.

I don't think your check helps with that. IIRC we always call
arch_perf_update_userpage() on the CPU the event actually runs on. So
it's always true.