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:

quoted

On Wed, Jun 24, 2020 at 01:31:42PM -0700, Sami Tolvanen wrote:

quoted

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 ?

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