[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. (Of course, that situation would improve if ftrace on x86-32 were ported to use -mfentry.) 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) - set -maccumulate-outgoing-args if CONFIG_JUMP_LABEL && gcc < 4.5.2 (for both 32-bit and 64-bit) -- Josh
