On Wed, 8 Mar 2017 15:29:59 -0600
Josh Poimboeuf <jpoim...@redhat.com> wrote:
> [adding Steven Rostedt to CC as an FYI]
>
> On Wed, Mar 08, 2017 at 10:25:01AM -0800, Linus Torvalds wrote:
> > On Wed, Mar 8, 2017 at 9:37 AM, Josh Poimboeuf <jpoim...@redhat.com> wrote:
> >
> > > - CONFIG_FUNCTION_GRAPH_TRACER sets it on x86-32 because of a gcc bug
> > > where the stack gets aligned before the mcount call. This issue
> > > should be mostly obsolete as most modern compilers now have -mfentry.
> > > We could make it dependent on CC_USING_FENTRY.
> >
> > Yeah. At some point we might even upgrade the compiler requirements to
> > no longer accept the mcount model.
>
> The plot slightly thickens...
>
> So I was mistaken about this problem not existing with newer versions of
> gcc, because the x86-32 ftrace code doesn't use -mfentry. It still
> relies on mcount. So CONFIG_FUNCTION_GRAPH_TRACER will still need
> -maccumulate-outgoing-args for *all* versions of gcc on x86-32.
OK, I admit, I was lazy here. I thought, who cares about x86-32
anymore ;-)
>
> (Of course, that situation would improve if ftrace on x86-32 were ported
> to use -mfentry.)
That can easily be done.
>
> Also, since -Os tells gcc to ignore -maccumulate-outgoing-args, this
> means that CONFIG_FUNCTION_GRAPH_TRACER with mcount needs a dependency
> on CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE.
>
> I suspect these issues also affect x86-64 with gcc 4.4.x and 4.5.x,
> which corresponds to the window after the funky DRAP prologue was
> introduced but before -mfentry was introduced.
>
> In summary, here are the changes I'm looking at:
>
> - set -maccumulate-outgoing-args if CONFIG_FUNCTION_GRAPH_TRACER &&
> !CC_USING_ENTRY
> (for both 32- and 64-bit)
>
> - somehow make CONFIG_FUNCTION_GRAPH_TRACER depend on either
> CC_USING_FENTRY or CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE
> (for both 32- and 64-bit)
>
> (not sure how to do that -- maybe just fail the build in the
> graph tracer + mcount + '-Os' case)
Could just place something like this in the x86 code:
#if defined(CONFIG_FUNCTION_GRAPH_TRACER) && \
!defined(CC_USING_FENTRY) && \
!defined(CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE)
# error Your compiler doesn't support function graph tracing
#endif
-- Steve
>
> - set -maccumulate-outgoing-args if CONFIG_JUMP_LABEL && gcc < 4.5.2
> (for both 32-bit and 64-bit)
>
