linux-kernel - Re: [PATCH] perf: make perf.data more self-descriptive (v4)

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Message-ID: <CABPqkBQWd=DyRRDhaxwDGrpCs=XqyTUv72+dn7GKsT=kjPqiJg@mail.gmail.com>
Date:	Mon, 12 Sep 2011 16:40:25 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	David Ahern <dsahern@...il.com>
Cc:	linux-kernel@...r.kernel.org, acme@...hat.com,
	peterz@...radead.org, mingo@...e.hu
Subject: Re: [PATCH] perf: make perf.data more self-descriptive (v4)

On Mon, Sep 12, 2011 at 4:33 PM, David Ahern <dsahern@...il.com> wrote:
>
> On 09/12/2011 07:54 AM, Stephane Eranian wrote:
>>> The problem with endianness handling I mentioned last week is related to
>>> the assumption that the HEADER_BUILD_ID is set. In my test case it is
>>> not (-B flag on perf-record) and that causes the aborts. A problem for
>>> another day which I can address it when I submit the patch to handle
>>> swapping for the feature mask.
>>>
>> Ok, now I understand your point better.
>>
>> The thing is that the endianess detection done by perf is currently broken.
>> It assumes that if the size of perf_event_attr saved in the file (attr_size)
>> does not match sizeof(perf_event_attr) then it must be because of endianess
>> issues. The reality is that it they can be different because the the ABI
>> has been extended (and I am sure it will), and this a perf tool may
>> have a larger struct perf_event_attr, than an old perf.data file, yet it
>> should be able to read it. We need to fix this issue now and not once
>> we hit the problem with extended ABI. I have a couple of ideas on this
>> and they revolve around using the magic prefix as the indicator of endianess
>> (and also detect older perf.data files).
>
> There is some handling of older perf data files. For example, data files
> without the adds_features mask are handled -- see
> perf_file_header__read(), if (header->size != sizeof(*header)) {}
>
Yes, that's quite possible. I am worried of attr_size vs.
sizeof(perf_event_attr).
I don't think this should be the test to detect endianess.


> The swapping of the adds_features mask is a mess because the mask was
> declared as an array of unsigned longs (util/include/linux/types.h).
>
> I do have it working now -- analyzing 32-bit PPC files on 32-bit and
> 64-bit x86 hosts.
>
> I have a patch to take a best guess on the feature mask. Your patchset
> actually helps there since the new features are on by default with no
> off switch. I guess to make it more robust I should add an endian
> feature mask and require it to be set rather than depending implicitly
> on the existence of other flags. Once Arnaldo picks up this patch I'll
> send out one for endianness handling of the features mask.
>
okay.
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