Messages in this thread | | | Date | Fri, 16 Sep 2011 08:50:21 -0600 | From | David Ahern <> | Subject | Re: [PATCH] perf: make perf.data more self-descriptive (v4) |
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On 09/16/2011 08:35 AM, Stephane Eranian wrote: > On Mon, Sep 12, 2011 at 4:43 PM, Peter Zijlstra <peterz@infradead.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?
on x86: $ od -c perf.data | head -1 0000000 P E R F F I L E h \0 \0 \0 \0 \0 \0 \0
on PPC: # od -c perf.data | head -1 0000000 P E R F F I L E \0 \0 \0 \0 \0 \0 \0 h
And then PPC file on x86: $ od -c perf-ppc.data | head -1 0000000 P E R F F I L E \0 \0 \0 \0 \0 \0 \0 h
So it is the same and that explains why the current code is:
if (readn(fd, header, sizeof(*header)) <= 0 || memcmp(&header->magic, __perf_magic, sizeof(header->magic))) return -1;
versus a u64 comparison. Printing magic as a PRIx64 running the command on x86:
ppc data file: header->magic 454c494646524550 x86 data file: header->magic 454c494646524550
Which is expected given the od -c output above.
David
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