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

From: Peter Maydell
Subject: Re: QEMU 32-bit vs. 64-bit binaries
Date: Tue, 10 May 2022 10:47:30 +0100

On Tue, 10 May 2022 at 10:31, Thomas Huth <thuth@redhat.com> wrote:
> On 10/05/2022 11.22, Dr. David Alan Gilbert wrote:
> > 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.

There's a lot of stuff you would want to chop out if you want
a stripped-down "just for KVM" binary, though, and not all of
it is 32-bit-related.  And unless you're chopping non-KVM
machine types and devices out of qemu-system-aarch64 anyway,
you need the 32-bit CPU support in there: we have a machine
type which has both 64-bit and 32-bit CPUs in it.

> 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.

No, Arm KVM is pretty restrictive -- effectively you can only really
use it with the 'virt' board. You can do a 32-bit guest, but you
do that with qemu-system-aarch64 and telling the 64-bit vcpu
"actually start with EL1 (guest kernel) in 32-bit mode".

-- PMM

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