linux-kernel - Re: [PATCH v2 03/10] riscv: Implement cmpxchg8/16() using Zabha

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Message-ID: <CAHVXubjCdse-3z3hKR81VpdvjxVaxPUZdmTwc4fvHordcfHVng@mail.gmail.com>
Date: Mon, 15 Jul 2024 14:56:34 +0200
From: Alexandre Ghiti <alexghiti@...osinc.com>
To: Andrea Parri <parri.andrea@...il.com>
Cc: Alexandre Ghiti <alex@...ti.fr>, Jonathan Corbet <corbet@....net>, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Albert Ou <aou@...s.berkeley.edu>, Nathan Chancellor <nathan@...nel.org>, 
	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 v2 03/10] riscv: Implement cmpxchg8/16() using Zabha

Hi Andrea,

On Wed, Jul 10, 2024 at 1:51 AM Andrea Parri <parri.andrea@...il.com> wrote:
>
> > > I admit that I found this all quite difficult to read; IIUC, this is
> > > missing an IS_ENABLED(CONFIG_RISCV_ISA_ZACAS) check.
> >
> > I'm not sure we need the zacas check here, since we could use a toolchain
> > that supports zabha but not zacas, run this on a zabha/zacas platform and it
> > would work.
>
> One specific set-up I was concerned about is as follows:
>
>   1) hardware implements both zabha and zacas
>   2) toolchain supports both zabha and zacas
>   3) CONFIG_RISCV_ISA_ZABHA=y and CONFIG_RISCV_ISA_ZACAS=n
>
> Since CONFIG_RISCV_ISA_ZABHA=y, the first asm goto will get executed
> and, since the hardware implements zacas, that will result in a nop.
> Then the second asm goto will get executed and, since the hardware
> implements zabha, it will result in the j zabha.  In conclusion, the
> amocas instruction following the zabha: label will get executed, thus
> violating (the semantics of) CONFIG_RISCV_ISA_ZACAS=n.  IIUC, the diff
> I've posted previously in this thread shared a similar limitation/bug.

So you mean that when disabling Zacas, we should actually disable
*all* the CAS instructions, even the Zabha ones. It makes sense and
allows for a single way to disable the CAS instructions but keeping
the other atomic operations.

I'll fix that and add a comment.

Thanks,

Alex

>
>   Andrea