linux-kernel - Re: [PATCH] riscv/atomic: Do proper sign extension also for unsigned in arch

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Message-Id: <CC2D9220-F8DE-4CC8-ACAD-7B1A21E276FE@jrtc27.com>
Date: Thu, 30 Jan 2025 17:21:28 +0000
From: Jessica Clarke <jrtc27@...c27.com>
To: Andreas Schwab <schwab@...e.de>
Cc: Alexandre Ghiti <alex@...ti.fr>,
 linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv/atomic: Do proper sign extension also for unsigned
 in arch_cmpxchg

On 30 Jan 2025, at 15:53, Andreas Schwab <schwab@...e.de> wrote:
> 
> On Jan 30 2025, Alexandre Ghiti wrote:
> 
>> a2 is used as it is passed by the calling function, so we can't be sure a2
>> is sign extended to me, what am I missing?
> 
> 32-bit scalar arguments are guaranteed to be sign extended on entry.

Firstly, the calling convention is irrelevant if the function is
inlined, which this almost always will be.

Secondly, whilst GCC (still?) maintains the invariant that 32-bit
registers are kept sign-extended in registers on RISC-V, and there’s a
non-normative note in the unprivileged spec about this, LLVM does not,
any extra bits are wholly unspecified, so you must not write inline
assembly that depends on them being well-define, either by ignoring the
bits or by converting your arguments to be whole-register types. If
your compiler has already extended the value in the manner you care
about then that’ll be a no-op, and if it hasn’t then it’ll do what you
need.

We had an issue like this years ago in FreeBSD (also for a uint32_t
CAS) and this was the conclusion I reached [1].

Jess

[1] https://reviews.freebsd.org/D22084