linux-kernel - Re: 16-bit store instructions &c?

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Message-Id: <b3512703-bab3-4999-ac20-b1b874fcfcc3@app.fastmail.com>
Date: Wed, 28 Aug 2024 13:48:41 +0000
From: "Arnd Bergmann" <arnd@...db.de>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: linux-kernel@...r.kernel.org, Linux-Arch <linux-arch@...r.kernel.org>
Subject: Re: 16-bit store instructions &c?

On Wed, Aug 28, 2024, at 13:11, Paul E. McKenney wrote:
> Hello, Arnd,
>
> You know how it goes, give them an inch...
>
> I did get a request for 16-bit xchg(), but last I checked, Linux still
> supports some systems that do not have 16-bit store instructions.
>
> Could you please let me know whether this is still the case?

Hi Paul,

The only one I'm aware of that can't do it easily
is a configuration on 32-bit ARM that enables both 
CONFIG_CPU_V6 and CONFIG_SMP, but I already wrote
a patch that forbids that configuration for other
reasons. I just need to send that patch.

There is a related problem with ARM RiscPC, which
uses a kernel built with -march=armv3, and that
disallows 16-bit load/store instructions entirely,
similar to how alpha ev5 and earlier lacked both
byte and word access.

Everything else that I see has native load/store
on 16-bit words and either has 16-bit atomics or
can emulate them using the 32-bit ones.

However, the one thing that people usually
want 16-bit xchg() for is qspinlock, and that
one not only depends on it being atomic but also
on strict forward-progress guarantees, which
I think the emulated version can't provide
in general.

This does not prevent architectures from doing
it anyway.

      Arnd