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

Pi 4 has much more throughput in 32-bit modes but the so
called experts of Debian decided to abandon it :-(

On 14.07.22 03:52, 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).
> Wookey

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