linux-kernel - Re: [PATCH 03/23] perf: Add ability to attach user level registers dump to sample

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Date:	Wed, 27 Jun 2012 17:13:44 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Jiri Olsa <jolsa@...hat.com>, acme@...hat.com,
	a.p.zijlstra@...llo.nl, mingo@...e.hu, paulus@...ba.org,
	cjashfor@...ux.vnet.ibm.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 03/23] perf: Add ability to attach user level registers
 dump to sample

On Wed, Jun 27, 2012 at 5:11 PM, Frederic Weisbecker <fweisbec@...il.com> wrote:
> On Tue, Jun 19, 2012 at 05:47:54PM +0200, Jiri Olsa wrote:
>> @@ -4061,6 +4105,19 @@ void perf_prepare_sample(struct perf_event_header *header,
>>               }
>>               header->size += size;
>>       }
>> +
>> +     if (sample_type & PERF_SAMPLE_REGS_USER) {
>> +             /* regs dump available bool */
>> +             int size = sizeof(u64);
>> +
>> +             data->regs_user = perf_sample_regs_user(regs);
>> +             if (data->regs_user) {
>> +                     u64 mask = event->attr.sample_regs_user;
>> +                     size += hweight64(mask) * sizeof(u64);
>> +             }
>> +
>> +             header->size += size;
>
> We'll need to remove the 64 bits registers for compat tasks but other than
> that, it looks ok.
You cannot do this. You cannot remove register values from under the hood.
The only way the user has to parse the sample is the sample_regs_users bitmask.
You have to return 0 for those unexisting regs for compat tasks.
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