Re: [PATCH 04/22] kbuild: lto: fix recordmcount

Peter Zijlstra
2020-06-25T07:46:28Z

On Wed, Jun 24, 2020 at 02:45:30PM -0700, Sami Tolvanen wrote:
> On Wed, Jun 24, 2020 at 11:27:37PM +0200, Peter Zijlstra wrote:
> > On Wed, Jun 24, 2020 at 01:31:42PM -0700, Sami Tolvanen wrote:
> > > With LTO, LLVM bitcode won't be compiled into native code until
> > > modpost_link. This change postpones calls to recordmcount until after
> > > this step.
> > >
> > > In order to exclude specific functions from inspection, we add a new
> > > code section .text..nomcount, which we tell recordmcount to ignore, and
> > > a __nomcount attribute for moving functions to this section.
> >
> > I'm confused, you only add this to functions in ftrace itself, which is
> > compiled with:
> >
> > KBUILD_CFLAGS = $(subst $(CC_FLAGS_FTRACE),,$(ORIG_CFLAGS))
> >
> > and so should not have mcount/fentry sites anyway. So what's the point
> > of ignoring them further?
> >
> > This Changelog does not explain.
>
> Normally, recordmcount ignores each ftrace.o file, but since we are
> running it on vmlinux.o, we need another way to stop it from looking
> at references to mcount/fentry that are not calls. Here's a comment
> from recordmcount.c:
>
> /*
> * The file kernel/trace/ftrace.o references the mcount
> * function but does not call it. Since ftrace.o should
> * not be traced anyway, we just skip it.
> */
>
> But I agree, the commit message could use more defails. Also +Steven
> for thoughts about this approach.

Ah, is thi because recordmcount isn't smart enough to know the
difference between "CALL $mcount" and any other RELA that has mcount?

At least for x86_64 I can do a really quick take for a recordmcount pass
in objtool, but I suppose you also need this for ARM64 ?