linux-kernel - Re: [PATCH v4 02/13] riscv: Do not fail to build on byte/halfword operations with Zawrs

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Message-ID: <ea55bb29-c3ba-4e71-a76a-f788c4a7ea16@redhat.com>
Date: Wed, 31 Jul 2024 12:27:02 -0400
From: Waiman Long <longman@...hat.com>
To: Alexandre Ghiti <alexghiti@...osinc.com>, Jonathan Corbet
 <corbet@....net>, Paul Walmsley <paul.walmsley@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Conor Dooley <conor@...nel.org>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Andrea Parri <parri.andrea@...il.com>, Nathan Chancellor
 <nathan@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
 Boqun Feng <boqun.feng@...il.com>, Arnd Bergmann <arnd@...db.de>,
 Leonardo Bras <leobras@...hat.com>, Guo Ren <guoren@...nel.org>,
 linux-doc@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
 linux-arch@...r.kernel.org
Subject: Re: [PATCH v4 02/13] riscv: Do not fail to build on byte/halfword
 operations with Zawrs

On 7/31/24 03:23, Alexandre Ghiti wrote:
> riscv does not have lr instructions on byte and halfword but the
> qspinlock implementation actually uses such atomics provided by the
> Zabha extension, so those sizes are legitimate.

Note that the native qspinlock code only need halfword atomic cmpxchg 
operation. However, if you also plan to use paravirtual qspinlock, you 
need to have byte-level atomic cmpxchg().

Cheers,
Longman

>
> Then instead of failing to build, just fallback to the !Zawrs path.
>
> Signed-off-by: Alexandre Ghiti <alexghiti@...osinc.com>
> ---
>   arch/riscv/include/asm/cmpxchg.h | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> index ebbce134917c..9ba497ea18a5 100644
> --- a/arch/riscv/include/asm/cmpxchg.h
> +++ b/arch/riscv/include/asm/cmpxchg.h
> @@ -268,7 +268,8 @@ static __always_inline void __cmpwait(volatile void *ptr,
>   		break;
>   #endif
>   	default:
> -		BUILD_BUG();
> +		/* RISC-V doesn't have lr instructions on byte and half-word. */
> +		goto no_zawrs;
>   	}
>   
>   	return;