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

[Laurent Vivier <laurent@vivier.eu>]

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