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

[Aurelien Jarno]

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 at aurel32.net                 http://www.aurel32.net
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