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

From: BALATON Zoltan
Subject: Re: QEMU 32-bit vs. 64-bit binaries
Date: Tue, 10 May 2022 12:14:14 +0200 (CEST)

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)



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