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) >