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

From: Dr. David Alan Gilbert
Subject: Re: QEMU 32-bit vs. 64-bit binaries
Date: Tue, 10 May 2022 10:22:36 +0100
User-agent: Mutt/2.2.1 (2022-02-19)

* 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.

ARM is a lot cleaner than x86; you don't suddenly find a little Cortex-M
machine with a big 64 bit core in it; yet on x86 our machines are
frankenstinian mixes with 25 year old chipsets and modern CPUs.


> -- PMM
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

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