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

[Laurent Vivier]

Le 04/12/2024 à 11:17, Daniel P. Berrangé a écrit :
> 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.

Now, there is another alternative: binfmt_misc is now part of user namespace, so you can define a 
binfmt rule for a given namespace:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=21ca59b365c091d583f36ac753eaa8baf947be6f

I have added a new parameter to unshare to help to run a new namespace with 
binfmt_misc:

https://github.com/util-linux/util-linux/commit/9d55de0d0d5c6298b38b58c3f4dc876c56213f85

Thanks,
Laurent