linux-kernel - Re: [PATCH] word-at-a-time: use the same return type for has

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Message-ID: <20230804130431.GA29929@willie-the-truck>
Date:   Fri, 4 Aug 2023 14:04:43 +0100
From:   Will Deacon <will@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] word-at-a-time: use the same return type for has_zero
 regardless of endianness

On Wed, Aug 02, 2023 at 08:17:32PM +0200, Arnd Bergmann wrote:
> On Wed, Aug 2, 2023, at 19:37, Linus Torvalds wrote:
> > On Wed, 2 Aug 2023 at 09:16, Nathan Chancellor <nathan@...nel.org> wrote:
> >>
> >> We see this warning with ARCH=arm64 defconfig + CONFIG_CPU_BIG_ENDIAN=y.
> >
> > Oh Christ. I didn't even realize that arm64 allowed a BE config.
> >
> > The config option goes back to 2013 - are there actually BE user space
> > implementations around?
> 
> At least NXP's Layerscape and Huawei's SoCs ended up in big-endian
> appliances, running legacy software ported from mips or powerpc.
> I agree this was a mistake, but that wasn't nearly as obvious ten
> years ago when there were still new BE-only sparc, mips and powerpc
> put on the market -- that really only ended in 2017.
> 
> > People, why do we do that? That's positively crazy. BE is dead and
> > should be relegated to legacy platforms. There are no advantages to
> > being different just for the sake of being different - any "security
> > by obscurity" argument would be far outweighed by the inconvenience to
> > actual users.
> >
> > Yes, yes, I know the aarch64 architecture technically allows BE
> > implementations - and apparently you can even do it by exception
> > level, which I had to look up. But do any actually exist?
> >
> > Does the kernel even work right in BE mode? It's really easy to miss
> > some endianness check when all the actual hardware and use is LE, and
> > when (for example) instruction encoding and IO is then always LE
> > anyway.
> 
> This was always only done for compatibility with non-portable
> software when companies with large custom network stacks argued
> that it was cheaper to build the entire open source software to
> big-endian than port their own product to little-endian. ;-)
> 
> We (Linaro) used to test all toolchain and kernel releases in
> big-endian mode as member companies had customers that asked
> for it, but that stopped a while ago as those legacy software
> stacks either got more portable or got replaced over time.
> 
> Many Arm systems won't boot BE kernels any more because UEFI
> firmware only supports LE, or because of driver bugs.
> Virtual machines are still likely to work fine though.
> I'm fairly sure that all Arm Cortex and Neoverse cores still\
> support BE mode in all exception levels, OTOH at least Apple's
> custom CPUs do not implement it at all.

Yes, that's right. The CPUs we have *do* tend to support BE, but
practically there isn't any software to run on them. I asked about
removing it a few years ago:

https://lore.kernel.org/linux-arm-kernel/20191011102747.lpbaur2e4nqyf7sw@willie-the-truck/

but Hanjun said that Huawei are using it, so it stayed.

Will