linux-kernel - Re: [PATCH 2/2] perf: SNB exclusive PMU access for INST_RETIRED:PREC

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Message-ID: <CABPqkBRhyHNiHvPLCQB3zOkH5oY9+Cn-mmMUp0rveBe+mg-z_Q@mail.gmail.com>
Date:	Mon, 22 Oct 2012 13:31:45 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andi Kleen <ak@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Jiri Olsa <jolsa@...hat.com>
Subject: Re: [PATCH 2/2] perf: SNB exclusive PMU access for INST_RETIRED:PREC_DIST

On Sun, Oct 21, 2012 at 8:03 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Stephane Eranian <eranian@...gle.com> wrote:
>
>> On Sun, Oct 21, 2012 at 6:55 PM, Ingo Molnar <mingo@...nel.org> wrote:
>> >
>> > * Andi Kleen <ak@...ux.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.
>> >
>> I assume by perf_attr:error_str, you actually mean:
>>
>> struct perf_event_attr {
>>    char error_str[PERF_ERR_LEN];
>> };
>>
>> Right?
>
> I don't think we should allocate space in the attr, instead we
> should use something like:
>
>         u8 __user       *err_str;
>         u32              err_str_len;
>
> which would be filled in by tooling with a string and a max_len
> value, and strncpy_to_user() could do the rest on the kernel
> side. [ A minor complication is that we don't have a
> strncpy_to_user() method at the moment. ]
>
> Static strings could be handled this way.
>
> [ Dynamic strings could be supported too with a few tricks,
>   although I doubt it matters in practice. ]
>
Ok, but this still limits returning error string to the perf_event_open()
syscall, not read(), ioctl() and such.

I am fine with this change. However, I think it should be added separately
from my inst_retired:prec_dist patch. It has a broader impact.


>> > 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?
>
> I think yes - and if not then it can be solved via some trivial
> offset value added to it on such an architecture, without
> complicating the code on normal architectures.
>
> Thanks,
>
>         Ingo
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