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

[Stephane Eranian]

On Sun, Oct 21, 2012 at 6:55 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Andi Kleen <ak@xxxxxxxxxxxxxxx> 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.
>
I assume by perf_attr:error_str, you actually mean:

struct perf_event_attr {
   char error_str[PERF_ERR_LEN];
};

Right?

> 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.
>
Is that always the case on all archs?

> 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
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