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SubjectRe: [PATCH] perf: make 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 | 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.


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