Re: QEMU 32-bit vs. 64-bit binaries

[BALATON Zoltan <balaton@eik.bme.hu>]

On Tue, 10 May 2022, Thomas Huth wrote:
> On 10/05/2022 11.22, Dr. David Alan Gilbert wrote:
>> * Peter Maydell (peter.maydell@linaro.org) wrote:
>>> On Tue, 10 May 2022 at 10:01, Thomas Huth <thuth@redhat.com> wrote:
>>>> 
>>>> On 10/05/2022 10.54, Markus Armbruster wrote:
>>>>> Thomas Huth <thuth@redhat.com> writes:
>>>>> 
>>>>> [...]
>>>>> 
>>>>>> I once suggested in the past already that we should maybe get rid of
>>>>>> the 32-bit variants in case the 64-bit variant is a full superset, so
>>>>>> we can save compile- and test times (which is quite a bit for QEMU),
>>>>>> but I've been told that the 32-bit variants are mostly still required
>>>>>> for supporting KVM on 32-bit host machines.
>>>>> 
>>>>> Do we still care for 32-bit host machines?
>>>> 
>>>> As long as the Linux kernel still supports 32-bit KVM virtualization, I
>>>> think we have to keep the userspace around for that, too.
>>>> 
>>>> But I wonder why we're keeping qemu-system-arm around? 32-bit KVM support
>>>> for ARM has been removed with Linux kernel 5.7 as far as I know, so I 
>>>> think
>>>> we could likely drop the qemu-system-arm nowadays, too? Peter, Richard,
>>>> what's your opinion on this?
>>> 
>>> Two main reasons, I think:
>>>   * command-line compatibility (ie there are lots of
>>>     command lines out there using that binary name)
>>>   * nobody has yet cared enough to come up with a plan for what
>>>     we want to do differently for these 32-bit architectures,
>>>     so the default is "keep doing what we always have"
>>> 
>>> In particular, I don't want to get rid of qemu-system-arm as the
>>> *only* 32-bit target binary we drop. Either we stick with what
>>> we have or we have a larger plan for sorting this out consistently
>>> across target architectures.
>> 
>> To my mind, qemu-system-arm makes a lot of sense, and I'd rather see the
>> 32 bit guests disappear from qemu-system-aarch64.
>> It's difficult to justify to someone running their aarch virt stack why
>> their binary has the security footprint that includes a camera or PDA.
>
> I'm not very familiar with KVM on ARM - but is it possible to use KVM there 
> with an arbitrary machine? If that's the case, a user might want to use KVM 
> on their 64-bit host to run a 32-bit guest machine, and then you need to keep 
> the 32-bit machines in the -aarch64 binary.
>
> Something like that is at least theoretically possible with ppc64, I think: 
> Using KVM-PR, it should be possible to run a g3beige (i.e. 32-bit) machine on 
> a 64-bit host. Not sure whether anybody has tried that in recent times (afaik 
> KVM-PR is in a rather bad shape nowadays), but it might have been possible at 
> one point in time in the past. (PPC folks, please correct me if I'm wrong)

https://www.talospace.com/2018/08/making-your-talos-ii-into-power-mac.html

Regards,
BALATON Zoltan