Re: [PATCH 0/1] meson: Deprecate 32-bit host systems

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 28/1/25 11:01, Philippe Mathieu-Daudé wrote:
> On 28/1/25 10:27, Daniel P. Berrangé wrote:
>> On Tue, Jan 28, 2025 at 10:17:33AM +0100, Philippe Mathieu-Daudé wrote:
>>> On 28/1/25 10:02, Alex Bennée wrote:
>>>> Thomas Huth <thuth@redhat.com> writes:
>>>>
>>>>> On 28/01/2025 01.42, Richard Henderson wrote:
>>>>>> Time for our biennial attempt to kill ancient hosts.
>>>>>> I've been re-working the tcg code generator a bit over the holidays.
>>>>>> One place that screams for a bit of cleanup is with 64-bit guest
>>>>>> addresses on 32-bit hosts.  Of course the best "cleanup" is to not
>>>>>> have to handle such silliness at all.
>>>>>> Two years after Thomas' last attempt,
>>>>>>      https://lore.kernel.org/qemu-devel/20230130114428.1297295-1- 
>>>>>> thuth@redhat.com/
>>>>>> which resulted only in deprecation of i686 host for system
>>>>>> emulation.
>>>>>> By itself, this just isn't enough for large-scale cleanups.
>>>>>> I'll note that we've separately deprecated mips32, set to expire
>>>>>> with the end of Debian bookworm, set to enter LTS in June 2026.
>>>>>> I'll note that there is *already* no Debian support for ppc32,
>>>>>> and that I am currently unable to cross-compile that host at all.
>>>>>
>>>>> IIRC the biggest pushback that I got two years ago was with regards to
>>>>> 32-bit arm: The recommended version of Raspberry Pi OS is still
>>>>> 32-bit:
>>>>>
>>>>>    https://lore.kernel.org/qemu-devel/ 
>>>>> F852C238-77B8-4E24-9494-8D060EB78F9F@livius.net/
>>>>>
>>>>> And looking at https://www.raspberrypi.com/software/operating-systems/
>>>>> this still seems to be the case...
>>>>>
>>>>> So I guess the main question is now: Would it be ok to kill support
>>>>> for 32-bit Raspberry Pi OS nowadays?
>>>>
>>>> I would argue yes for a few reasons.
>>>>
>>>>     - you can't buy 32 bit only Pi's AFAICT, even the Pi Zero 2W can 
>>>> work
>>>>       with a 64 bit OS.
>>>>
>>>>     - It's not like the versions shipping in bullseye and bookworm will
>>>>       stop working.
>>>>
>>>>     - Even if we deprecate now there will likely be one more Debian
>>>>       release cycle that gets 32 bit host support.
>>>>
>>>>>> Showing my hand a bit, I am willing to limit deprecation to
>>>>>> 64-bit guests on 32-bit hosts.  But I'd prefer to go the whole hog:
>>>>>> unconditional support for TCG_TYPE_I64 would remove a *lot* of
>>>>>> 32-bit fallback code.
>>>>
>>>> I support going the whole hog. I would be curious what use cases still
>>>> exist for an up to date 32-on-32 QEMU based emulation?
>>>
>>> Current maintainers don't have spare time to support the 32-on-32
>>> emulation. If there is interest in the community for such niche,
>>> someone needs to step forward, willing to maintain it.
>>
>> I'm not sure that's the case here.
>>
>> 32-on-32 is already effectively unmaintained, so we're not suffering
>> in terms of keeping the 32-on-32 code reliable.
>>
>> We're suffering from the complexity that 32-on-32 code places on the
>> rest of the XX-on-64 code that we do care about.
>>
>> IOW if someone volunteered to maintain 32-on-32 that's not actually
>> solving the complexity problem, just perpetuating it.
>>
>> The current maintainers only interested in XX-on-64 will still suffer
>> ongoing burden from the code complexity caused by 32-on-32 merely
>> existing.
>>
>> So again lets be clear...
>>
>> Either we...
>>
>>   * ...want to kill 32-on-32 code to reduce the complexity on the
>>     main XX-on-64 codebase regardless of interest in 32-on-32
>>
>> Or
>>
>>   * ...want to kill 32-on-32 code because it is buggy due to lack
>>     of maintainers, but would welcome someone to step forward to
>>     maintain it
>>
>> It sounded like we were wanting the former, not the latter.
> 
> Yes, we want to former. But as Thomas pointed out, last time
> someone showed up, and while the maintainers weren't willing to
> keep 32-on-32 [*], they kept maintaining it at the price of restricting
> XX-on-64.
> 
> [*] back then we proved system emulation XX-on-32 wasn't really useful
> anymore, and user emulation 64-on-32 was partly broken, so only
> 32-on-32 user emulation was functional.

So it seems reasonable to deprecate and ask interested 32-on-32 user
emulation users to use QEMU 10.1 release.