Re: [PATCH 2/2] perf: SNB exclusive PMU access for INST_RETIRED:PREC_DIST

[Ingo Molnar <mingo@kernel.org>]

* Andi Kleen <ak@linux.intel.com> wrote:

> > > > This isn't limited to admin, right? So the above turns into a DoS on the
> > > > console.
> > > >
> > > Ok, so how about a WARN_ON_ONCE() instead?
> > 
> > That should be fine I guess ;-)
> 
> imho there is need for a generic mechanism to return an error 
> string to the user program without such hacks.

Agreed - we could return an 'extended errno' long error return 
value, which in reality is a pointer to an error string (in 
perf_attr::error_str), and copy that string to user-space at 
perf syscall return time.

Thus error-string aware tooling could print the error string.

So PMU drivers could do something obvious like:

	return (long)"perf: INST_RETIRED.PREC_DIST only works in exclusive mode";

The perf syscall notices these pointers by noticing that the 
error code returned is outside the errno range.

Old userspace will get a -EINVAL and no string copied into the 
error string buffer.

New userspace would get the error string copied into 
perf_attr::error_str, plus a 'normal' -EINVAL error code.

The only cost on the kernel side is to make sure all "string 
errors" are returned as long.

Thanks,

	Ingo