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

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Date:	Sat, 21 Nov 2015 18:41:42 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Brian Robbins <brianrob@...rosoft.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH] perf: Fallback to JIT support for mmap'd non-ELF
 binaries.

Em Fri, Nov 20, 2015 at 01:18:55PM -0800, Brian Robbins escreveu:
> 
> 
> 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.
> > 
> > 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?
> > 
> > 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.

Its in my queue, AFAIK there were not objections expressed, I'll try to
get this reviewed/merged soon.

- Arnaldo
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