Re: [PATCH] perf: Fallback to JIT support for mmap'd non-ELF binaries.
From: Arnaldo Carvalho de Melo
Date: Sat Nov 21 2015 - 16:41:52 EST
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.
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