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

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Message-ID: <CABPqkBShe6OG1ukQ=7vn_Mr=Aa_c8jcbo1mYuF+41Euy5_R7Dw@mail.gmail.com>
Date:	Fri, 16 Sep 2011 16:35:34 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	David Ahern <dsahern@...il.com>, linux-kernel@...r.kernel.org,
	acme@...hat.com, mingo@...e.hu
Subject: Re: [PATCH] perf: make perf.data more self-descriptive (v4)

On Mon, Sep 12, 2011 at 4:43 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, 2011-09-12 at 16:40 +0200, Stephane Eranian wrote:
>> I don't think this should be the test to detect endianess.
>
> You should be able to tell the endianness from the PERF_MAGIC string, it
> stores the string as a u64, so depending on endianness it reads back as
> either: PERFFILE or ELIFFREP or whatever the bswap64 result is.
>

I believe in big endian, if you do od -c perf.data | head -1, you also see:

0000000   P   E   R   F   F   I   L   E   h  \0  \0  \0  \0  \0  \0  \0

static const char *__perf_magic = "PERFFILE";
#define PERF_MAGIC      (*(u64 *)__perf_magic)

u64 hm = PERF_MAGIC;

The layout in memory is the same for both little-endian and
big-endian. Thus the layout on the file is the same.

When you look at the memory as u64, then things are different:
In little-endian, hm=0x454c494646524550
in big-endian,   hm=0x5045524646494c45

In big-endian, the MSB 0x50 ('P') ends up at the lowest memory address.
In little-endian, the LSB 0x50 ('P') ends up at the lowest memory address.

Thus, I suspect we need to write in the file a different MAGIC for big vs.
little endian.

David, can you confirm this?
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