Re: [PATCH] rust: enable fentry support

On Mon, Jul 27, 2026 at 08:33:35PM +0200, Miguel Ojeda wrote:

On Mon, Jul 27, 2026 at 7:18 PM Paul Murphy [off-list ref] wrote:

quoted

Hrm, s390x has more nuanced usage. It still uses -mrecord-mcount and
-mnop-mcount. The x86 configurations I've looked at use the kernel's
tooling to nop and record as needed. Is there any reason s390x cannot
use those, or should rustc support those (for s390x only)?

Cc'ing the s390x maintainers -- they probably know why they use them.

Cc'ing the ftrace maintainers.

I do not think this is true for x86 in general. Generating the
__fentry__ call and recording its location are two separate operations.
-Zinstrument-mcount=fentry only generates the call. Dynamic ftrace also
needs the call site recorded in __mcount_loc section.

The recording mechanism is selected in kernel/trace/Kconfig:

config FTRACE_MCOUNT_USE_CC
   def_bool y
   depends on $(cc-option,-mrecord-mcount)
   depends on !FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY
   depends on DYNAMIC_FTRACE

config FTRACE_MCOUNT_USE_OBJTOOL
   def_bool y
   depends on HAVE_OBJTOOL_MCOUNT
   depends on !FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY
   depends on !FTRACE_MCOUNT_USE_CC                <--- NOTICE THAT --
   depends on DYNAMIC_FTRACE
   select OBJTOOL

config FTRACE_MCOUNT_USE_RECORDMCOUNT
   def_bool y
   depends on !FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY
   depends on !FTRACE_MCOUNT_USE_CC
   depends on !FTRACE_MCOUNT_USE_OBJTOOL
   depends on DYNAMIC_FTRACE

There are currently three ways to generate __mcount_loc section:

1. The compiler does it with -mrecord-mcount.
2. Objtool does it with --mcount.
3. scripts/recordmcount does it as a post-processing step.

These are selected in that order.

Looking on my system:

For x86 GCC builds - current GCC supports -mrecord-mcount,
so FTRACE_MCOUNT_USE_CC is selected and the compiler gets

  -pg -mfentry -mrecord-mcount

For x86 Clang builds - current x86 Clang does not support -mrecord-mcount,
so FTRACE_MCOUNT_USE_OBJTOOL is selected. Clang gets

  -pg -mfentry

and objtool gets

  --mcount --mnop

s390 uses the compiler path with both current GCC and Clang:

  -pg -mfentry -mrecord-mcount -mnop-mcount

Here -mrecord-mcount creates __mcount_loc and -mnop-mcount emits the
six-byte nop expected by the s390 ftrace code.

So -Zinstrument-mcount=fentry alone is sufficient only when some
later build step records the generated call, as objtool does for
x86 Clang FTRACE_MCOUNT_USE_OBJTOOL builds. It is not sufficient for
FTRACE_MCOUNT_USE_CC. For FTRACE_MCOUNT_USE_RECORDMCOUNT - the
scripts/recordmcount post-processing step is also currently wired to
the C build rule rather than the Rust build rule.

Objtool mcount recording is currently supported only by x86 and powerpc.
Those are the only architectures which select HAVE_OBJTOOL_MCOUNT. And
s390 currently has no objtool support.

So I do not think -mrecord-mcount Rust support is only needed for
s390. The Rust build path needs an equivalent of -mrecord-mcount when
FTRACE_MCOUNT_USE_CC is selected. And additionally an equivalent of
-mnop-mcount for s390 (this is debatable, we could check if this brings
us much or we could drop it).