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Re: armhf kernels on arm64 hardware

On Fri, Jul 15, 2022 at 05:13:33PM +0100, Wookey wrote:
> Ah thanks Paul. I was wondering why we were being accused of 'Debian
> abandonning armhf' when it was news to me, and I'm just writing the
> 'ARM ports status' talk for Debconf next week.
> Clearly one normally does not run foreign-arch kernels on hardware so
> we don't have to support it, and Ben is right to say 'this is not a
> bug'.
> On the other hand, if the armhf kernel does work on RPi4 with a few
> config options, and there is an actual use case, then the question is
> what is the downside of enabling the config options?
> Does this only work for the RPi4, or does it enable/prevent 32-bit kernels on other 64-bit machines?

Certainly people have been running 32 bit kernels on the Pi 3 and 4 and
it works fine.  Some high end aarch64 CPUs don't support 32 bit mode,
but that is certainly not the case for the Pi's CPU.

> Do i386 kernels work on amd64 machines?

Yes, but...  They certainly don't work with more than 3.5GB or so of ram
unless you use the pae version of the kernel then you can have some more.
There have been issues in some cases with systems that had too much ram
where rather than just ignoring it the kernel would fail to boot.

Of course it was very common a 15 or 20 years ago to run debian i386 with
an amd64 kernel and was fully supported by debian including the installer
which as far as I remember even recommended that kernel if supported by
the host.  Quite a bit of user space code still had issues with 64 bit,
but you got to run a kernel that could take full advantage of your ram
and other cpu features, while running 32 bit user space (since the amd64
kernel of course can run i386 binaries just fine).

For example this was very much in Debian:

So an amd64 kernel in the i386 archive.

> Sounds like something that might be worth discussion at debconf next week. I'll mention it in the talk.

Well it would essentially mean treating arm like i386 used to be treated.
It is certainly not a thing Debian hasn't supported before.

Len Sorensen

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