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

[Andy Lutomirski <luto@amacapital.net>]

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.


On a semi-related note: would this all be nicer if there were vdso
function __u64 __vdso_perf_event__read_count(int fd, void *userpage)?
This is very easy to do nowadays.  If we got *really* fancy, it would
be possible to have an rdpmc_safe in the vdso, which has some
benefits, although it would be a bit evil and wouldn't work if
userspace tracers like pin are in use.

--Andy