Are machines available to general public?

Yes. IBM continues to sell and maintain the architecture. The most current products are z196/z114. They are not cheap, however.

Is full source available?

Yes, the kernel flavour for s390x was already integrated into the s390 port and in fact you can only run the older port on s390x machines anyway, since Squeeze. All changes IBM did to gcc etc. are upstream and already available in Debian.

It supersedes s390 post-Wheezy, i.e. s390 is being dropped in favour of the new port.

You can still build s390 packages on a s390x buildd (and vice versa), so we can reuse the existing machines.

Are there 3 or more developers (or n-ms) actively maintaining the port? Who are they?

What sort of architecture is this?

S/390 is the architecture of the IBM mainframes since decades. 64bit support was introduced in 2000, and all z* machines have it.

Does it have any users? If an embedded system are there real systems shipping that a Debian port will be useful for?

The target audience are the users of the existing s390 port. The community is small, the user base is also small. There are companies, however, using it in production, and a bunch of people using it for play machines.

The port can also be emulated using Hercules, which is in Debian.

Is there kernel and toolchain support? At what level? Are the latest versions supported, or are legacy releases required for compatibility with some hardware?

s390x is fully supported in both the kernel and the GNU toolchain.

Has the ABI stabilised, or are there major ABI changes coming up?

Yes, ABI is stable.

How do you install a system?

The goal is to use the same debian-installer as for s390 (just needs porting to behave the same like on s390). Which means mostly netinst.

Has a buildd been setup? How much of the archive has been built (count by source package, builds of old versions are fine for this case)?

The current s390 buildd (zandonai) is also building s390x for debian-ports. There had been a second buildd to get the port up and running. There will hopefully be a real second buildd soon.

90% of the archive have been built, it's keeping up with 100% for now. (See the Debian buildd stats page for details.)

The other packages are mostly held up by general FTBFS'es in unstable.

What hardware is potentially available as a fast buildd?

The z10 VMs at ZIVIT we already have. Potentially a z990 VM, and potentially another z10 VM.

Is there any corporate support of this arch, and the Debian port in particular?

IBM supports the architecture. The Debian port does not have corporate support at this time.

Is there an example box developers can login to to see if it works?

Not yet. We aim at providing a s390x chroot on the s390 porterbox zelenka as soon as the port is integrated. (DSA does not want debian-ports porter chroots on their boxes.)

It's also worth considering whether the port has any special requirements.

None at this time.