linux-kernel - Re: [PATCH v2 1/6] powerpc/perf: Define big-endian version of perf_mem_data

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Message-ID: <20170307102359.GE6515@twins.programming.kicks-ass.net>
Date:   Tue, 7 Mar 2017 11:23:59 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>
Cc:     mpe@...erman.id.au, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org,
        Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Wang Nan <wangnan0@...wei.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v2 1/6] powerpc/perf: Define big-endian version of
 perf_mem_data_src

On Tue, Mar 07, 2017 at 03:28:17PM +0530, Madhavan Srinivasan wrote:
> 
> 
> On Monday 06 March 2017 04:52 PM, Peter Zijlstra wrote:
> >On Mon, Mar 06, 2017 at 04:13:08PM +0530, Madhavan Srinivasan wrote:
> >>From: Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
> >>
> >>perf_mem_data_src is an union that is initialized via the ->val field
> >>and accessed via the bitmap fields. For this to work on big endian
> >>platforms, we also need a big-endian represenation of perf_mem_data_src.
> >Doesn't this break interpreting the data on a different endian machine?
> 
> IIUC, we will need this patch to not to break the interpreting data
> on a different endian machine. Data collected from power8 LE/BE
> guests with this patchset applied. Kindly correct me if I missed
> your question here.

So your patch adds compile time bitfield differences. My worry was that
there was no dynamic conversion routine in the tools (it has for a lot
of other places).

This yields two questions:

 - are these two static layouts identical? (seeing that you illustrate
   cross-endian things working this seems likely).

 - should you not have fixed this in the tool only? This patch
   effectively breaks ABI on big-endian architectures.