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

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Sat, May 09, 2026 at 11:47:45PM -0700, Namhyung Kim wrote:
> 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.

Can I take this as an Reviewed-by or Acked-by?

- Arnaldo
 
> 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