Re: [PATCH v2] arm64: compat: Implement misalignment fixups for multiword loads

From: Aurelien Jarno <aurel32@debian.org>
To: Ard Biesheuvel <ardb@kernel.org>,
	linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com,
	arnd@arndb.de, will@kernel.org, mark.rutland@arm.com,
	maz@kernel.org, broonie@kernel.org, debian-arm@lists.debian.org,
	Vagrant Cascadian <vagrant@debian.org>,
	Riku Voipio <riku.voipio@iki.fi>,
	Steve McIntyre <steve@einval.com>
Subject: Re: [PATCH v2] arm64: compat: Implement misalignment fixups for multiword loads
Date: Tue, 16 Aug 2022 21:28:53 +0200	[thread overview]
Message-ID: <Yvvv9dzJ4kxTCo0L@aurel32.net> (raw)
In-Reply-To: <20220714015214.GP27816@mail.wookware.org>

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Hi,

On 2022-07-14 02:52, Wookey wrote:
> On 2022-07-01 15:53 +0200, Ard Biesheuvel wrote:
> > The 32-bit ARM kernel implements fixups on behalf of user space when
> > using LDM/STM or LDRD/STRD instructions on addresses that are not 32-bit
> > aligned.
> 
> > This feature is one of the remaining impediments to being able to switch
> > to 64-bit kernels on 64-bit capable hardware running 32-bit user space,
> > so let's implement it for the arm64 compat layer as well.
>  
> > Note to cc'ees: if this is something you would like to see merged,
> > please indicate so. This stuff is unlikely to get in if there are no
> > users.
> 
> Decent 32-bit arm hardware is thin on the ground these days. Debian
> still has some but it's getting old and flaky. Being able to build
> reliably on 64-bit hardware is important and useful. Unaligned
> accesses are much less of a problem than they used to be, but they can
> still happen, so having these fixups available is definitely a good
> thing.
> 
> Debian runs its 32-bit buildds with alignment fixups turned on. It
> looks like the boxes still hit about 1 per day.
> 
> We also do 32 bit builds on 64-bit kernels (in 32-bit userspaces) and
> it mostly works. We do have packages that fail on 64-bit kernels and
> have to be built on real 32-bit hardware, but I don't know how much of
> that would be fixed by this patch. Some, presumably.
> 
> So yes, cheers for this. It is helpful in the real world (or at least
> it should be).

I confirm that this would be very helpful to Debian, so that 32-bit
binaries behaves the same with a 32-bit or a 64-bit kernel. Otherwise we
need to keep running (old) 32-bit hardware.

What's the status of those patches?

Thanks
Aurelien

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@aurel32.net                 http://www.aurel32.net

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