Messages in this thread | | | Date | Tue, 12 Jan 2021 16:32:30 +0100 | From | Peter Zijlstra <> | Subject | Re: [RFC] perf/x86: Only expose userspace rdpmc for events on current CPU |
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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?
> 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.
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