Re: [RFC 5/5] x86,perf: Only allow rdpmc if a perf_event is mapped

[Andy Lutomirski]

On Sun, Oct 19, 2014 at 2:35 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Thu, Oct 16, 2014 at 05:00:56PM -0700, Andy Lutomirski wrote:
>> The current cap_user_rdpmc code seems rather confused to me.  On x86,
>> *all* events set cap_user_rdpmc if the global rdpmc control is set.
>> But only x86_pmu events define .event_idx, so amd uncore events won't
>> actually expose their rdpmc index to userspace.
>>
>> Would it make more sense to add a flag PERF_X86_EVENT_RDPMC_PERMITTED
>> that gets set on all events created while rdpmc == 1, to change
>> x86_pmu_event_idx to do something like:
>>
>> if (event->hw.flags & PERF_X86_EVENT_RDPMC_PERMITTED)
>>   return event->hw.event_base_rdpmc + 1;
>> else
>>   return 0;
>>
>> and to change arch_perf_update_userpage cap_user_rdpmc to match
>> PERF_X86_EVENT_RDPMC_PERMITTED?
>>
>> Then we could ditch the static key and greatly simplify writes to the
>> rdpmc flag by just counting PERF_X86_EVENT_RDPMC_PERMITTED events.
>>
>> This would be a user-visible change on AMD, and I can't test it.
>
> I have AMD hardware to test this. But yes something like that seems
> fine.

Before I totally screw this up: is .event_idx used for anything except
userspace rdpmc?  There are a whole bunch of implementations of that
callback:

 - perf_event_idx_default seems fishy
 - power_pmu_event_idx seems even fishier
 - cpumsf_pmu_event_idx is the same as power_pmu_event_idx.
 - perf_swevent_event_idx returns 0.

etc.

x86 is the only implementation of arch_perf_update_userpage, which
makes me think that the .event_idx callback should just be removed and
that arch_perf_update_userpage should be responsible for filling it in
if needed.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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