Message-ID: <4F344990.7050303@gmail.com>
Date: Thu, 09 Feb 2012 15:32:48 -0700
From: David Ahern <dsahern@gmail.com>
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To: Stephane Eranian <eranian@google.com>, acme@redhat.com
CC: linux-kernel@vger.kernel.org, peterz@infradead.org, mingo@elte.hu,
        robert.richter@amd.com, ming.m.lin@intel.com, andi@firstfloor.org,
        asharma@fb.com, ravitillo@lbl.gov, vweaver1@eecs.utk.edu,
        khandual@linux.vnet.ibm.com
Subject: Re: [PATCH v6 14/18] perf: fix endianness detection in perf.data
References: <1328826068-11713-1-git-send-email-eranian@google.com> <1328826068-11713-15-git-send-email-eranian@google.com>
In-Reply-To: <1328826068-11713-15-git-send-email-eranian@google.com>
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On 02/09/2012 03:21 PM, Stephane Eranian wrote:
> 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.
> 
> Signed-off-by: Stephane Eranian <eranian@google.com>

Acked-by: David Ahern <dsahern@gmail.com>

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