Re: [PATCH] binfmt: Don't consider riscv{32,64} part of the same family

[Daniel P . Berrangé]

On Tue, Dec 03, 2024 at 07:57:14AM -0600, Richard Henderson wrote:
> On 12/3/24 04:35, Peter Maydell wrote:
> > On Tue, 3 Dec 2024 at 10:19, Daniel P. Berrangé <berrange@redhat.com> wrote:
> > > Separatley this from patch, we should also consider whether
> > > it is time to do the same for aarch64/arm7.
> > > 
> > > If I look at this page:
> > > 
> > >    https://gpages.juszkiewicz.com.pl/arm-socs-table/arm-socs.html
> > > 
> > > and sort by 'announced' to see msot recent CPUs first, then
> > > almost all of them have "NO" in the "aarch32 support" column.
> > > 
> > > IOW, on modern aarch64 CPUs, qemu-arm is the only viable way
> > > to run 32-bit usermode binaries AFAICT, and suggests we ought
> > > to be creating a binfmt rule for that on aarch64 hosts.
> > 
> > What happens if you have a host CPU that *does* support 32-bit
> > natively and you also register the binfmt rule? Does the
> > host kernel prefer to execute natively or does it invoke
> > QEMU? I don't think we want to roll out something that
> > silently downgrades native execution to emulation...
> 
> The registered rule applies and the kernel invokes qemu.

This is all quiet difficult from a distro POV, but not QEMU's fault.

We want to install the binfmt rules in a way that we "do the right thing"
regardless of hardware out of the box.

The systemd logic for loading binfmt rules is unconditional, loading
everything from /usr/lib/binfmt.d, but we need a way to make things
conditional on the lack of support for aarch32 on the currently running
platform.

With regards,
Daniel
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