Re: [PATCH v3 05/11] riscv: Implement arch_cmpxchg128() using Zacas

From: "Arnd Bergmann" <arnd@arndb.de>
To: "Conor.Dooley" <conor.dooley@microchip.com>,
	"Alexandre Ghiti" <alexghiti@rivosinc.com>
Cc: "Andrew Jones" <ajones@ventanamicro.com>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Conor Dooley" <conor@kernel.org>,
	"Rob Herring" <robh@kernel.org>,
	"Krzysztof Kozlowski" <krzk+dt@kernel.org>,
	"Andrea Parri" <parri.andrea@gmail.com>,
	"Nathan Chancellor" <nathan@kernel.org>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Ingo Molnar" <mingo@redhat.com>, "Will Deacon" <will@kernel.org>,
	"Waiman Long" <longman@redhat.com>,
	"Boqun Feng" <boqun.feng@gmail.com>,
	"Leonardo Bras" <leobras@redhat.com>, guoren <guoren@kernel.org>,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-riscv@lists.infradead.org,
	Linux-Arch <linux-arch@vger.kernel.org>
Subject: Re: [PATCH v3 05/11] riscv: Implement arch_cmpxchg128() using Zacas
Date: Thu, 18 Jul 2024 11:35:33 +0200	[thread overview]
Message-ID: <da057e38-a3e1-42d8-8b7d-4e2dfed6d7b7@app.fastmail.com> (raw)
In-Reply-To: <20240718-stammer-envy-f77637a8d039@wendy>

On Thu, Jul 18, 2024, at 10:33, Conor Dooley wrote:
> On Thu, Jul 18, 2024 at 09:48:42AM +0200, Alexandre Ghiti wrote:
>> On Wed, Jul 17, 2024 at 10:34 PM Andrew Jones <ajones@ventanamicro.com> wrote:
>> > On Wed, Jul 17, 2024 at 08:19:51AM GMT, Alexandre Ghiti wrote:
>> > > +
>> > > +union __u128_halves {
>> > > +     u128 full;
>> > > +     struct {
>> > > +             u64 low, high;
>> >
>> > Should we consider big endian too?
>> 
>> Should we care about big endian? We don't deal with big endian
>> anywhere in our kernel right now.
>
> There's one or two places I think that we do actually have some
> conditional stuff for BE. The Zbb string routines I believe is one such
> place, and maybe there are one or two others. In general I'm not of the
> opinion that it is worth adding complexity for BE until there's
> linux-capable hardware that supports it (so not QEMU or people's toy
> implementations), unless it's something that userspace is able to see.

I don't think you want to go there at all: maintaining an
extra user space ABI (or two if you add 32-bit BE as well)
has a huge long-term cost, and there is pretty much zero
benefit for a BE ABI these days.

Adding it to arm64 turned out to be a mistake. We did have
a handful of users in the first year, and it technically
still works, but I don't think there are any users left
after they managed to fix their nonportable legacy
userspace from that was ported from big-endian mips or
powerpc.

     Arnd

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