linux-kernel - Re: [PATCH 02/17] perf: Add ability to attach user level registers dump to sample

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Message-ID: <CABPqkBQz88k4Srw+QLu-7OWC=xGtg1DDNFMiRfN10vRsZHVZ0w@mail.gmail.com>
Date:	Thu, 26 Jul 2012 19:42:55 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	acme@...hat.com, a.p.zijlstra@...llo.nl, mingo@...e.hu,
	paulus@...ba.org, cjashfor@...ux.vnet.ibm.com, fweisbec@...il.com,
	gorcunov@...nvz.org, tzanussi@...il.com, mhiramat@...hat.com,
	robert.richter@....com, fche@...hat.com,
	linux-kernel@...r.kernel.org, masami.hiramatsu.pt@...achi.com,
	drepper@...il.com, asharma@...com, benjamin.redelings@...cent.org
Subject: Re: [PATCH 02/17] perf: Add ability to attach user level registers
 dump to sample

On Wed, Jul 25, 2012 at 8:27 PM, Jiri Olsa <jolsa@...hat.com> wrote:
> On Wed, Jul 25, 2012 at 07:39:18PM +0200, Stephane Eranian wrote:
>> On Sun, Jul 22, 2012 at 2:14 PM, Jiri Olsa <jolsa@...hat.com> wrote:
>
> SNIP
>
>> > +       if (sample_type & PERF_SAMPLE_REGS_USER) {
>> > +               u64 avail = (data->regs_user != NULL);
>> > +
>> > +               /*
>> > +                * If there are no regs to dump, notice it through
>> > +                * first u64 being zero.
>> > +                */
>> > +               perf_output_put(handle, avail);
>> > +
>> The only role of avail is to report whether or not you've captured actual
>> registers. Could it be used to report the sampled process ABI (32 vs. 64)
>> instead? Something like:
>>       PERF_SAMPLE_REGS_ABI_NONE -> no regs captured (emulate your
>> current behavior)
>>       PERF_SAMPLE_REGS_ABI_32 -> 32 bit ABI regs captured
>>       PERF_SAMPLE_REGS_ABI_64 -> 64 bit ABI regs captured
>>
>> That could help the tools interpret the register values.
>
> Yes, I think that could help once we start dealing with compat tasks.
>
You don't control whether or not you capture compat tasks. So
you have to deal with those right now.

> The current userspace code stays untouched, because it checks for
> 'avail != 0', which stays even with your change.
>
> I think this could be sent later with all other fixes I'm already
> working on. But I can work/send it preferentially before whole patchset
> is taken if you like.
>
Well, why not do it now. You'd have to rename the available field
into something more sensible. Also need to prepare it for future
extension if they ever become necessary.
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