Date: Tue, 23 Oct 2012 15:57:26 +0200
From: Andi Kleen <andi@firstfloor.org>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andi Kleen <andi@firstfloor.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org, acme@redhat.com, eranian@google.com,
        Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH 05/34] perf, x86: Report PEBS event in a raw format
Message-ID: <20121023135726.GG16230@one.firstfloor.org>
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On Tue, Oct 23, 2012 at 03:50:50PM +0200, Peter Zijlstra wrote:
> PERF_SAMPLE_RAW has a u32 size header and starts the data after that.
> This means you PERF_SAMPLE_RAW output ends up on a u32 aligned end
> address -- assuming the data is a u64 multiple, this is not good.
> 
> > It appears to work at least.
> 
> It would on x86, I'm fairly sure it'll break on things like SPARC. We
> used to have checks in the userspace code to warn for this on x86 as
> well. Not sure if that's still there.

Ok this is x86 specific code.

> 
> Hmm, so in kernel/events/core.c:perf_prepare_sample() there's a
> WARN_ON_ONCE() in the PERF_SAMPLE_RAW branch that should trigger with
> this.

Haven't seen this this warning so far.

If I added padding the perf userland would need to learn about it too,
I suspect it would need all the ugly macros like NETLINK. I always hated
those.

-Andi
-- 
ak@linux.intel.com -- Speaking for myself only.
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