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

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Message-ID: <4E736C5A.1040502@gmail.com>
Date:	Fri, 16 Sep 2011 09:33:46 -0600
From:	David Ahern <dsahern@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Stephane Eranian <eranian@...gle.com>
CC:	linux-kernel@...r.kernel.org, acme@...hat.com, mingo@...e.hu
Subject: Re: [PATCH] perf: make perf.data more self-descriptive (v4)



On 09/16/2011 09:19 AM, Peter Zijlstra wrote:
> On Fri, 2011-09-16 at 08:50 -0600, David Ahern wrote:
>>
>> ppc data file: header->magic 454c494646524550
>> x86 data file: header->magic 454c494646524550
>>
> Ok then we need to fix that magic thing..

Right, but it breaks backwards/forwards compatibility. e.g.,

diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
index 73fa59e..603a8bb9 100644
--- a/tools/perf/util/header.c
+++ b/tools/perf/util/header.c
@@ -65,9 +65,7 @@ char *perf_header__find_event(u64 id)
    return NULL;
 }

-static const char *__perf_magic = "PERFFILE";
-
-#define PERF_MAGIC (*(u64 *)__perf_magic)
+#define PERF_MAGIC 0x5045524646494c45ULL  // "PERFFILE"

 struct perf_file_attr {
    struct perf_event_attr  attr;

That puts the magic in the file based on endianness of the recording
system. e.g., on x86:

$ od -c perf-magic.data | head -1
0000000   E   L   I   F   F   R   E   P   h  \0  \0  \0  \0  \0  \0  \0

Which means older perf binaries can't analyze new files (created with
new magic order) and newer perf binaries can't analyze older perf files
-- the new command will incorrectly want to do endian swapping on the
old file.

David
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