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

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Date:   Tue, 27 Jul 2021 08:46:23 -0700
From:   Yonghong Song <yhs@...com>
To:     Pavo Banicevic <pavo.banicevic@...tura.hr>,
        <linux@...linux.org.uk>, <ast@...nel.org>, <daniel@...earbox.net>,
        <andrii@...nel.org>, <kafai@...com>, <songliubraving@...com>,
        <john.fastabend@...il.com>, <kpsingh@...nel.org>,
        <nathan@...nel.org>, <ndesaulniers@...gle.com>,
        <ivan.khoronzhuk@...aro.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>,
        <bpf@...r.kernel.org>, <clang-built-linux@...glegroups.com>,
        <matt.redfearn@...s.com>, <mingo@...nel.org>,
        <dvlasenk@...hat.com>, <juraj.vijtiuk@...tura.hr>,
        <robert.marko@...tura.hr>, <luka.perkov@...tura.hr>,
        <jakov.petrina@...tura.hr>
Subject: Re: [PATCH 1/3] arm: include: asm: swab: mask rev16 instruction for
 clang



On 7/27/21 7:11 AM, Pavo Banicevic 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.
> 
> 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

What if the kernel is compiled with clang compiler?
Does this still work?

To workaround samples/bpf issue, I think you can use
__BPF_TRACING__ macro.

>   
>   static inline __attribute_const__ __u32 __arch_swab32(__u32 x)
>   {
>