Messages in this thread | | | Date | Tue, 12 Jan 2021 18:03:45 +0100 | From | Peter Zijlstra <> | Subject | Re: [RFC] perf/x86: Only expose userspace rdpmc for events on current CPU |
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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@infradead.org> 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.
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