Re: [PATCH] gcov,x86: Mark GCOV broken for x86

[Nick Desaulniers]

On Mon, Jun 14, 2021 at 9:20 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Mon, Jun 14, 2021 at 09:05:04AM -0700, Nick Desaulniers wrote:
> > On Mon, Jun 14, 2021 at 3:17 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > >
> > >
> > > As recently discovered, there is no function attribute to disable the
> > > -fprofile-generate instrumentation. As such, GCOV is fundamentally
> > > incompatible with architectures that rely on 'noinstr' for correctness.
> >
> > Is there context for comment, or is this patch meant as a joke?
>
> Only if you think recursion in exception entry code is funny.
>
> noinstr *MUST* disable any and all compiler generated instrumentation,
> currently it that isn't the case for -fprofile-gnerate, nor
> -fprofile-arc.
>
> Look for all the fun we had with KCOV back then. Luckily KCOV
> instrumentation was trivial to patch out using objtool, so that's what
> x86 is currently doing.
>
> Luckily both compilers grew a __no_sanitize_coverage recently and we no
> longer have to rely on objtool fixing up the compiler output for much
> longer.
>
>   https://lkml.kernel.org/r/20210527194448.3470080-1-elver@xxxxxxxxxx
>
> Now all we need is one more such attribute to kill -fprofile-* stuff.

__attribute__((no_instrument_function)) is already wired to not emit
calls to mcount()/fentry().  I think extending it to also apply to
coverage (-fprofile-arcs) and instrumentation based profiling
(-fprofile-generate) is reasonable.

-- 
Thanks,
~Nick Desaulniers