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

[Peter Zijlstra <peterz@infradead.org>]

On Sun, Oct 19, 2014 at 03:57:54PM -0700, Andy Lutomirski wrote:
> > Maybe, but at that point we commit to yet another ABI... I'd rather just
> > put a 'sane' implementation in a library or so.
> 
> This cuts both ways, though.  For vdso timekeeping, the underlying
> data structure has changed repeatedly, sometimes to add features, and
> sometimes for performance, and the vdso has done a good job insulating
> userspace from it.  (In fact, until 3.16, even the same exact kernel
> version couldn't be relied on to have the same data structure with
> different configs, and even now, no one really wants to teach user
> libraries how to parse the pvclock data structures.)

Fair enough, but as it stands we've already committed to the data
structure exposed to userspace.

> I would certainly not suggest putting anything beyond the bare minimum
> into the vdso.

Depends on what you really want to do I suppose, if you've got a pinned
event and know there cannot be multiplexing, not doing the time reads
the multiplications and all that saves a ton of cycles. But in generic I
suppose you have to do all that.

> FWIW, something should probably specify exactly when it's safe to try
> a userspace rdpmc.  I think that the answer is that, for a perf event
> watching a pid, only that pid can do it (in particular, other threads
> must not try).  For a perf event monitoring a whole cpu, the answer is
> less clear to me.

This all was really only meant to be used for self-monitoring, so where
an event is attached to the very same task, anything else and I'm find
disabling it.