On Wed, Jun 21, 2023 at 08:32:00PM +0100, Adam Sampson via Dng wrote:
> Didier Kryn <kryn@???> writes:
>
> >> # chroot /mnt /bin/bash
> >
> > Maybe I'm going crazy but this is absolutely impossible. You must not
> > only enter the new filesystem, you must also change the cpu. You need
> > qemu for this, certainly not a bare chroot.
>
> It's not impossible, but it does require some setup first. There's a
> userspace-only version of qemu that translates target-machine system
> calls into native system calls, and you can set up Linux's support for
> custom binary formats so qemu-user gets invoked automatically when you
> try to run a non-native executable. The emulation's not as good as
> qemu's full-machine emulation (in particular, it tends to have limited
> support for recently-added system calls), but it usually works well
> enough for bootstrapping.
>
> The Debian wiki has some notes on how to set it up:
> https://wiki.debian.org/QemuUserEmulation

Using plain multiarch setup did indeed come into play fairly recently;
might have been 2017 or so when I "discovered" it. Before that one
needed a complete filesystem with a qemu-static setup, or, as Didier
suggests, a separate VM already in the target architecture.

We, toddlers of today, use multiarch; it's two simple commands:

# dpkg --add-architecture armhf
# apt-get update

and then the (amd64) system is all setup to run armhf binaries via the
automatic emulation (using binfmt I believe). You do not need an armhf
kernel to run armhf binaries.

Obviously those binaries are executed with respect to the host machine
hardware which very likely is different from the target machine
hardware. But it's generally good enough for both spinning up a target
filesystem, and for building a kernel for a target filesystem. (The
latter is partly made possible with how target hardware nowadays is
separated out with DTB declarations, which provide hardeware ABI
descriptions for kernel software).

Thus, following those two commands, you may spin up a target
filesystem, e.g., the way I did it:

# debootstrap --variant=minbase --no-merge-usr --arch armhf \
    chimaera /mnt http://deb.devuan.org

That was they way I set up the two armhf build hosts I'm providing for
devuan package building.

Some hardware modules I got from a near-miss Armbian version; this was
a year ago but afair my hosts are 805 cpus that needed something
special for networking.

You might have similar success with getting a working Armbian
boot+filesystem for your hardware and then just replace the filesystem
with a Devuan debootstrapped filesystem. It's easier if you don't need
to compile the kernel yourself.

Ralph.

>

> -- 
> Adam Sampson <ats@???>                         <http://offog.org/>
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