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

[Thomas Huth <thuth@redhat.com>]

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)

  Thomas