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

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Message-ID: <Zo3NBHUEMMec/6uD@andrea>
Date: Wed, 10 Jul 2024 01:51:32 +0200
From: Andrea Parri <parri.andrea@...il.com>
To: Alexandre Ghiti <alex@...ti.fr>
Cc: 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>,
	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

> > 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.

  Andrea