Re: [RFC][PATCH] objtool,x86_64: Replace recordmcount with objtool

[Sami Tolvanen]

On Fri, Jun 26, 2020 at 4:29 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Thu, Jun 25, 2020 at 03:40:42PM -0700, Sami Tolvanen wrote:
>
> > > Not boot tested, but it generates the required sections and they look
> > > more or less as expected, ymmv.
>
> > > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > > index a291823f3f26..189575c12434 100644
> > > --- a/arch/x86/Kconfig
> > > +++ b/arch/x86/Kconfig
> > > @@ -174,7 +174,6 @@ config X86
> > >     select HAVE_EXIT_THREAD
> > >     select HAVE_FAST_GUP
> > >     select HAVE_FENTRY                      if X86_64 || DYNAMIC_FTRACE
> > > -   select HAVE_FTRACE_MCOUNT_RECORD
> > >     select HAVE_FUNCTION_GRAPH_TRACER
> > >     select HAVE_FUNCTION_TRACER
> > >     select HAVE_GCC_PLUGINS
> >
> > This breaks DYNAMIC_FTRACE according to kernel/trace/ftrace.c:
> >
> >   #ifndef CONFIG_FTRACE_MCOUNT_RECORD
> >   # error Dynamic ftrace depends on MCOUNT_RECORD
> >   #endif
> >
> > And the build errors after that seem to confirm this. It looks like we might
> > need another flag to skip recordmcount.
>
> Hurm, Steve, how you want to do that?

Steven, did you have any thoughts about this? Moving recordmcount to
an objtool pass that knows about call sites feels like a much cleaner
solution than annotating kernel code to avoid unwanted relocations.

Sami