linux-kernel - Re: [PATCH] perf: Fallback to JIT support for mmap'd non-ELF binaries.

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Message-ID: <CABPqkBRci99KYP3_6=3BqhEk-8SkAbgao_b-FQJe3cDcAHE2PA@mail.gmail.com>
Date:	Mon, 23 Nov 2015 09:36:44 -0800
From:	Stephane Eranian <eranian@...gle.com>
To:	Brian Robbins <brianrob@...rosoft.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] perf: Fallback to JIT support for mmap'd non-ELF binaries.

On Fri, Nov 20, 2015 at 1:18 PM, Brian Robbins <brianrob@...rosoft.com> wrote:
>
>
> On Fri, 20 Nov 2015, Peter Zijlstra wrote:
>
>> On Thu, Nov 19, 2015 at 11:45:45PM +0000, Brian Robbins wrote:
>>
>> > Thank you for the feedback.  The file format is similar to PE, but is
>> > not identical.  So, we would be implementing something very scoped,
>> > which doesn't feel right to me.
>>
>> *groan* you just had to go and invent yet another executable format,
>> right? :-)
> Nah, it just has some extra stuff in it that makes this less desirable -
> it's technically PE, but anyway.
>>
>> > I am interested in the new JIT support, however my understanding from
>> > the information that I've read is that it requires kernel support in
>> > 4.x, though I can't seem to find where I read that.  I want to make
>> > sure that this works on older kernels (3.x) as well.
>>
>> As I think Stephane explained, this is only required if you need to
>> match up kernel and userspace timestamps, which is important for dynamic
>> code generation, less so for static code in a weird format.
>
> Yes, agreed.
>
>>
>> So what the new JIT stuff does is online write 'fake' ELF files with
>> symbol sections and (optionally?) dwarf debug info for line numbers.
>>
The debug info section is optional and contains the line table info. This
can be used should you want to display source code along with assembly.

>> Since you don't dynamically generate code, you can offline generate
>> these ELF files and redirect the symbol parser bits to that (we already
>> look for debug ELF files in various locations), or...
>>
>> > The reason I went with this approach is because it is simple for
>> > runtimes to implement and has no requirement that perf understand the
>> > file format.  I am open to feedback if there is a preferred solution
>> > that would still work for older kernels as well.
>>
>> Since, someone somewhere needs to go parse this funny new file format
>> anyhow to either generate /tmp files or fake ELF files or whatever, you
>> might as well put that decoder in perf?
>>
If you have a new binary format, I think you'd have to write the parser in perf.

>> Or just ship these fake ELF files in /usr/lib/debug/ or whatever the
>> 'right' location for the distro at hand is.
>>
>
> This seems like a reasonable approach.
>
> Stephane, are your changes available for public consumption?  Last I
> recall, the patches were still in review.
>
I will post V8 this week.
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