Re: [PATCH 1/3] perf tools: Allow vmlinux to fallback to kallsyms on NO_LIBELF=1

[Namhyung Kim <namhyung@kernel.org>]

On Mon, 10 Nov 2014 09:11:41 -0300, Arnaldo Carvalho de Melo wrote:
> Em Mon, Nov 10, 2014 at 03:53:06PM +0900, Namhyung Kim escreveu:
>> On Fri, 7 Nov 2014 12:29:31 -0300, Arnaldo Carvalho de Melo wrote:
>> > Em Fri, Nov 07, 2014 at 02:20:04PM +0900, Namhyung Kim escreveu:
>> >> When libelf is not used, perf cannot not show symbol names since it
>> >> doesn't access the ELF symbol table.  But kernel is different as it
>> >> can fallback to kallsyms.
>
>> >> It worked w/o libelf, but recent change to use vmlinux for kernel
>> >> symbols break it.
>
>> >> With this change, it now can show kernel symbols again:
>
>> > Ok, but since you added that minimal ELF symtab loading, isn't better to
>> > try that first, i.e. if we find a vmlinux file with a build-id and with
>> > symbols in it...
>
>> > If that isn't the case, i.e. no vmlinux was found, then we fallback to
>> > kallsyms, to check if that is available.
>
>> > I.e. with your new minimalistic ELF symtab loader if we have a suitable
>> > vmlinux but no kallsyms, we end up resolving no symbols even having that
>> > nice vmlinux :-)
>> 
>> Yeah, maybe.  But it'd add a substantial complexity also and I tried to
>> make it simple and small only to show usual userspace symbols.
>> 
>> I think that about a half of the complexity of the dso__load_sym() in
>> symbol-elf.c came from the kernel (and module) support.  And expecting
>
> Right, I agree that thing grew too complex, reducing that complexity is
> something we should do when we have the chance. I.e. perhaps we could
> separate kernel/modules handling out of it, even if just at source code
> level at first...

Hmm.. okay.  I'll try.

Thanks,
Namhyung


>
>> kallsyms on the system is not a high barrier IMHO.  So I decided to just
>> fall back to kallsyms for kernel dsos.  Mayby we can add the kernel
>> support incrementally later.
>
> The suggestion came because you did that minimalistic reader, which if
> can be used for vmlinux files, would be great, as resolving vmlinux
> samples is such a major usecase :-)