Re: [PATCH v5 14/18] perf: fix endianness detection in perf.data

[David Ahern]

On 02/06/2012 11:17 AM, Arnaldo Carvalho de Melo wrote:
> Em Thu, Feb 02, 2012 at 01:54:44PM +0100, Stephane Eranian escreveu:
>> The current version of perf detects whether or not
>> the perf.data file is written in a different endianness
>> using the attr_size field in the header of the file. This
>> field represents sizeof(struct perf_event_attr) as known
>> to perf record. If the sizes do not match, then perf tries
>> the byte-swapped version. If they match, then the tool assumes
>> a different endianness.
>>
>> The issue with the approach is that it assumes the size of
>> perf_event_attr always has to match between perf record and
>> perf report. However, the kernel perf_event ABI is extensible.
>> New fields can be added to struct perf_event_attr. Consequently,
>> it is not possible to use attr_size to detect endianness.
>>
>> This patch takes another approach by using the magic number
>> written at the beginning of the perf.data file to detect
>> endianness. The magic number is an eight-byte signature.
>> It's primary purpose is to identify (signature) a perf.data
>> file. But it could also be used to encode the endianness.
>>
>> The patch introduces a new value for this signature. The key
>> difference is that the signature is written differently in
>> the file depending on the endianness. Thus, by comparing the
>> signature from the file with the tool's own signature it is
>> possible to detect endianness. The new signature is "PERFILE2".
>>
>> Backward compatiblity with existing perf.data file is
>> ensured.
> 
> Looks ok, but IIRC David Ahern interacted with you on this specific
> patch in the past, having his Acked-by and/or Tested-by would be great,
> David?

I don't recall anything changing since version 4. I scanned over the
change and it all looks familiar; testing worked fine as well -- I was
able to analyze a PPC file on x86 and vice versa.

Acked-by: David Ahern <dsahern@xxxxxxxxx>
Tested-by: David Ahern <dsahern@xxxxxxxxx>


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