Re: [PATCH 2/2] perf build: compile BPF skeletons with -mcpu=v3

[Namhyung Kim <namhyung@kernel.org>]

Hi Arnaldo,

On Thu, May 07, 2026 at 11:49:59PM +0000, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] Adding `-mcpu=v3` to `CLANG_OPTIONS` forces all BPF skeletons to use the BPF v3 ISA, breaking backward compatibility for perf tools designed to run on older kernels.
> --
> 
> commit 2e8791aeccf694e1ae9ed9fda46e3a6f73332838
> Author: Suchit Karunakaran <suchitkarunakaran@gmail.com>
> 
> perf build: compile BPF skeletons with -mcpu=v3
> 
> This commit adds -mcpu=v3 to CLANG_OPTIONS to support the atomic
> cmpxchg instruction required by the lock_contention BPF program.
> This forces Clang to emit BPF ISA v3 instructions during skeleton
> compilation.
> 
> > diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
> > index cee19c923c060..a9a8c84b6b006 100644
> > --- a/tools/perf/Makefile.perf
> > +++ b/tools/perf/Makefile.perf
> 
> [ ... ]
> 
> > @@ -1195,7 +1195,7 @@ ifneq ($(CROSS_COMPILE),)
> >  CLANG_TARGET_ARCH = --target=$(notdir $(CROSS_COMPILE:%-=%))
> >  endif
> >  
> > -CLANG_OPTIONS = -Wall
> > +CLANG_OPTIONS = -Wall -mcpu=v3
> 
> Does this change break backward compatibility for other perf tools running
> on older kernels?
> 
> Since CLANG_OPTIONS is used in the pattern rule for compiling all BPF
> skeletons, this forces every BPF program in perf (such as
> augmented_raw_syscalls.bpf.c and off_cpu.bpf.c) to target the BPF ISA v3.
> 
> Kernels older than 5.1 do not support v3 instructions and will reject
> these programs during the verifier load phase.

I think it's old enough and we can move on to ensure v3 instructions.

Thanks,
Namhyung

> 
> Should the -mcpu=v3 flag be restricted exclusively to the target that
> needs it, perhaps using a target-specific append for lock_contention.bpf.o?
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260507184238.65280-1-suchitkarunakaran@gmail.com?part=2