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Date:   Tue, 03 Jan 2023 17:50:00 +0100
From:   "Arnd Bergmann" <arnd@arndb.de>
To:     "Mark Rutland" <mark.rutland@arm.com>,
        "Peter Zijlstra" <peterz@infradead.org>
Cc:     "Boqun Feng" <boqun.feng@gmail.com>,
        "Linus Torvalds" <torvalds@linux-foundation.org>,
        "Jonathan Corbet" <corbet@lwn.net>,
        "Will Deacon" <will@kernel.org>,
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        "Tejun Heo" <tj@kernel.org>, "Christoph Lameter" <cl@linux.com>,
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Subject: Re: [RFC][PATCH 05/12] arch: Introduce arch_{,try_}_cmpxchg128{,_local}()
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On Tue, Jan 3, 2023, at 17:19, Mark Rutland wrote:
> On Tue, Jan 03, 2023 at 02:03:37PM +0000, Mark Rutland wrote:
>> On Tue, Jan 03, 2023 at 01:25:35PM +0000, Mark Rutland wrote:
>> > On Tue, Dec 20, 2022 at 12:08:16PM +0100, Peter Zijlstra wrote:

>> ... makes GCC much happier:
>
>> ... I'll go check whether clang is happy with that, and how far back that can
>> go, otherwise we'll need to blat the high half with a separate constaint that
>> (ideally) doesn't end up allocating a pointless address register.
>
> Hmm... from the commit history it looks like GCC prior to 5.1 might not be
> happy with that, but that *might* just be if we actually do arithmetic on the
> value, and we might be ok just using it for memroy effects. I can't currently
> get such an old GCC to run on my machines so I haven't been able to check.

gcc-5.1 is the oldest (barely) supported compiler, the minimum was
last raised from gcc-4.9 in linux-5.15. If only gcc-4.9 and older are
affected, we're good on mainline but may still want a fix for stable
kernels.

I checked that the cross-compiler binaries from [1] still work, but I noticed
that this version is missing the native aarch64-to-aarch64 compiler (x86 to
aarch64 and vice versa are there), and you need to install libmpfr4 [2]
as a dependency. The newer compilers (6.5.0 and up) don't have these problems.

     Arnd

[1] https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/arm64/5.5.0/
[2] http://ftp.uk.debian.org/debian/pool/main/m/mpfr4/libmpfr4_3.1.5-1_arm64.deb