linux-kernel - Re: [PATCH 2/7] riscv: Implement cmpxchg8/16() using Zabha

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Message-ID: <CAHVXubj1RpN80f0vNQwFtaNKORnw2F3yzA1_0txa3A+9-usdFw@mail.gmail.com>
Date: Wed, 29 May 2024 14:29:02 +0200
From: Alexandre Ghiti <alexghiti@...osinc.com>
To: Andrea Parri <parri.andrea@...il.com>
Cc: Jonathan Corbet <corbet@....net>, Paul Walmsley <paul.walmsley@...ive.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>, 
	Waiman Long <longman@...hat.com>, 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, 
	linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org, 
	linux-arch@...r.kernel.org
Subject: Re: [PATCH 2/7] riscv: Implement cmpxchg8/16() using Zabha

On Wed, May 29, 2024 at 1:54 AM Andrea Parri <parri.andrea@...il.com> wrote:
>
> > +zabha:                                                                       \
> > +     __asm__ __volatile__ (                                          \
> > +             prepend                                                 \
> > +             "       amocas" cas_sfx " %0, %z2, %1\n"                \
> > +             append                                                  \
> > +             : "+&r" (r), "+A" (*(p))                                \
> > +             : "rJ" (n)                                              \
> > +             : "memory");                                            \
>
> Couldn't a platform have Zabha but not have Zacas?  I don't see how this
> asm goto could work in such case, what am I missing?

Zabha amocas.[b|h] instructions are only implemented if Zacas is
present, as the specification states: "If Zacas [2] extension is also
implemented, Zabha further provides the AMOCAS.[B|H] instructions."

But the code you mention is only for 8 and 16bit operations, so I
think we are good anyway?

Thanks,

Alex

>
>   Andrea