linux-kernel - Re: [PATCH 5/7] kcov: add ioctl(KCOV_UNIQUE

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Message-ID: <CAG_fn=UrJGBcmqYkaqy3qckg=vVQZ4fA2cwruSnCdphkP0ZoZQ@mail.gmail.com>
Date: Wed, 23 Apr 2025 15:16:48 +0200
From: Alexander Potapenko <glider@...gle.com>
To: Marco Elver <elver@...gle.com>
Cc: quic_jiangenj@...cinc.com, linux-kernel@...r.kernel.org, 
	kasan-dev@...glegroups.com, Aleksandr Nogikh <nogikh@...gle.com>, 
	Andrey Konovalov <andreyknvl@...il.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Dmitry Vyukov <dvyukov@...gle.com>, 
	Ingo Molnar <mingo@...hat.com>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 5/7] kcov: add ioctl(KCOV_UNIQUE_ENABLE)

> > >  void notrace __sanitizer_cov_trace_pc_guard(u32 *guard)
> > >  {
> > > -       if (!check_kcov_mode(KCOV_MODE_TRACE_PC, current))
> > > -               return;
> > > +       u32 pc_index;
> > > +       enum kcov_mode mode = get_kcov_mode(current);
> > >
> > > -       sanitizer_cov_write_subsequent(current->kcov_state.s.trace,
> > > -                                      current->kcov_state.s.trace_size,
> > > -                                      canonicalize_ip(_RET_IP_));
> > > +       switch (mode) {
> > > +       case KCOV_MODE_TRACE_UNIQUE_PC:
> > > +               pc_index = READ_ONCE(*guard);
> > > +               if (unlikely(!pc_index))
> > > +                       pc_index = init_pc_guard(guard);
> >
> > This is an unlikely branch, yet init_pc_guard is __always_inline. Can
> > we somehow make it noinline? I know objtool will complain, but besides
> > the cosmetic issues, doing noinline and just giving it a better name
> > ("kcov_init_pc_guard") and adding that to objtool whilelist will be
> > better for codegen.
>
> I don't expect it to have a big impact on the performance, but let's
> check it out.

Oh wait, now I remember that when we uninline that function, that
introduces a register spill in __sanitizer_cov_trace_pc_guard, which
we want to avoid.