linux-kernel - Re: [PATCH 1/3] arm: include: asm: swab: mask rev16 instruction for clang

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Message-ID: <CAEf4Bza2zZK2m4fmDUXKoURxMmUcfr8gvLR9wxF1vFPBmc2gHA@mail.gmail.com>
Date:   Tue, 27 Jul 2021 14:43:39 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Pavo Banicevic <pavo.banicevic@...tura.hr>,
        Arnd Bergmann <arnd@...aro.org>, linux@...linux.org.uk,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        matt.redfearn@...s.com, Ingo Molnar <mingo@...nel.org>,
        dvlasenk@...hat.com, Juraj Vijtiuk <juraj.vijtiuk@...tura.hr>,
        robert.marko@...tura.hr, Luka Perkov <luka.perkov@...tura.hr>,
        Jakov Petrina <jakov.petrina@...tura.hr>
Subject: Re: [PATCH 1/3] arm: include: asm: swab: mask rev16 instruction for clang

On Tue, Jul 27, 2021 at 10:53 AM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Tue, Jul 27, 2021 at 7:12 AM Pavo Banicevic
> <pavo.banicevic@...tura.hr> wrote:
> >
> > From: Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
> >
> > The samples/bpf with clang -emit-llvm reuses linux headers to build
> > bpf samples, and this w/a only for samples (samples/bpf/Makefile
> > CLANG-bpf).
> >
> > It allows to build samples/bpf for arm bpf using clang.
> > In another way clang -emit-llvm generates errors like:
> >
> > CLANG-bpf  samples/bpf/tc_l2_redirect_kern.o
> > <inline asm>:1:2: error: invalid register/token name
> > rev16 r3, r0
> >
> > This decision is arguable, probably there is another way, but
> > it doesn't have impact on samples/bpf, so it's easier just ignore
> > it for clang, at least for now.
>
> NACK
>
> The way to fix these is to sort out the header includes, not turning
> off arbitrary things that are used by the actual kernel build for 32b
> ARM.

Would it be too horrible to just get rid of `clang -emit-llvm` and use
vmlinux.h (we don't need to do CO-RE, btw, just generate vmlinux.h
from the matching kernel)? Kumar has already started moving in that
direction in his recent patch set ([0]). Would that get rid of all
these issues?

  [0] https://patchwork.kernel.org/project/netdevbpf/list/?series=519281&state=*


>
> >
> > Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>
> > ---
> >  arch/arm/include/asm/swab.h | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/arch/arm/include/asm/swab.h b/arch/arm/include/asm/swab.h
> > index c6051823048b..a9fd9cd33d5e 100644
> > --- a/arch/arm/include/asm/swab.h
> > +++ b/arch/arm/include/asm/swab.h
> > @@ -25,8 +25,11 @@ static inline __attribute_const__ __u32 __arch_swahb32(__u32 x)
> >         __asm__ ("rev16 %0, %1" : "=r" (x) : "r" (x));
> >         return x;
> >  }
> > +
> > +#ifndef __clang__
> >  #define __arch_swahb32 __arch_swahb32
> >  #define __arch_swab16(x) ((__u16)__arch_swahb32(x))
> > +#endif
> >
> >  static inline __attribute_const__ __u32 __arch_swab32(__u32 x)
> >  {
> > --
> > 2.32.0
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers