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

[Peter Zijlstra <peterz@infradead.org>]

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.