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Date:   Wed, 4 Jan 2023 11:36:18 +0000
From:   Mark Rutland <mark.rutland@arm.com>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>, dennis@kernel.org,
        Tejun Heo <tj@kernel.org>, Christoph Lameter <cl@linux.com>,
        Heiko Carstens <hca@linux.ibm.com>, gor@linux.ibm.com,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        borntraeger@linux.ibm.com, svens@linux.ibm.com,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "David S . Miller" <davem@davemloft.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, joro@8bytes.org,
        suravee.suthikulpanit@amd.com, Robin Murphy <robin.murphy@arm.com>,
        dwmw2@infradead.org, baolu.lu@linux.intel.com,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
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        Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: [RFC][PATCH 05/12] arch: Introduce
 arch_{,try_}_cmpxchg128{,_local}()
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On Tue, Jan 03, 2023 at 05:50:00PM +0100, Arnd Bergmann wrote:
> 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.

Yup; I just wanted something that would easily backport to stable, at least as
far as linux-4.9.y (where I couldn't find the minimum GCC version when I looked
yesterday).

> 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.

I was trying the old kernel.org crosstool binaries, but I was either missing a
library (or I have an incompatible version) on my x86_64 host. I'll have
another look today -- thanks for the pointers!

Mark.

>      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