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

From: Wookey <wookey@wookware.org>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: 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: Thu, 14 Jul 2022 02:52:15 +0100	[thread overview]
Message-ID: <20220714015214.GP27816@mail.wookware.org> (raw)
In-Reply-To: <20220701135322.3025321-1-ardb@kernel.org>

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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).

Wookey
-- 
Principal hats:  Debian, Wookware, ARM
http://wookware.org/

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