The Penguin and the Dinosaur
The short answer is, yes, it's Linux. The version I'm running at penguinvm.princeton.edu is 2.2.13. The S/390 patches are already in the stock 2.2.14 source, and IBM's team is hard at work integrating L/390 into 2.3.x. By the time you read this, it should all have been merged fairly seamlessly into the mainline development kernel tree.
Linux on the System/390 is just as much Linux as Linux/PPC, Linux/SPARC, Linux/m68k or Linux/Alpha. The System/390 port is integrated into the main kernel source tree; unlike ELKS or RT-Linux, it's the stock kernel, rather than a subset or an extension. From a user's perspective, nothing differs between running it on a 390 and running it on an x86 machine. The shell works the same way, X applications work the same way, GNOME has been ported (by adding a couple of lines to the configure scripts) and it all just works. There's not much exciting to say, because if it comes with source, the odds are very good you can build and run it with minimal effort. As “Think Blue Linux” gets off the ground, you should be able to install a binary with RPM. At the time of this writing, they had about 420 RPMs available; there should soon be many more.
From an application programmer's perspective, it's about the same. There are a couple of very minor patches for config.sub and config.guess to get configure to recognize the S/390 architecture (these have already been submitted to Ben Elliston at Red Hat, who maintains autoconf). With its next release, configure should support L/390 with no modification. Once the patches have been applied, anything which uses autoconf builds and runs right out of the box. This includes Bochs (an x86 emulator); apparently, some intrepid soul has built Bochs and booted the NT Server, very slowly, under L/390.
The other major issue is endianness. Unlike the x86, S/390 is big-endian. Programs which assume Intel byte order will fail. But if these programs have already been ported to, say, Linux/PPC (another 32-bit, big-endian platform), then those bugs have been crushed. This is not a 390-specific problem as such, but a general portability issue.
At the time of this writing, pthreads support was still buggy in the version of L/390 I was using, which caused odd bugs in VNC and the Hercules emulator; this has apparently already been fixed, but I haven't rebuilt the system with the latest patches. Likewise, there was an optimizer bug in GCC, causing it to generate bad code with -O2 and above; this too has already been fixed, and should no longer be an issue by the time this appears in print.
Deep within the guts of the kernel, things get fairly hairy. IBM has released its kernel code and glibc modifications under the GPL, and they're available from the IBM DeveloperWorks page; it's all there and well-commented. It's fascinating stuff, especially the huge differences between x86 interrupt design and the way S/390 talks to its peripherals, but far beyond the scope of this article.
Networking is handled via either the OSA-2 (Ethernet or token-ring) device driver or point-to-point high-speed channels. (OSA is the ironically named “open systems adapter”: the device driver is OCO—object code only, meaning no source.) If you're running under VM, you can set up something called IUCV (Inter User Communications Vehicle), which basically lets you establish a PPP connection to another virtual machine. If, by the way, you're beginning to get the feeling IBM has its own language, all acronyms, you're absolutely right. What this IUCV means is that it's purely internal—there is no real-world device corresponding to an IUCV adapter. You can also set up a virtual CTCA interface (channel-to-channel adapter). Channels are real-world interfaces, implemented these days over fiber optic cables. IUCV runs at 500MBps, CTCA at 250MBps. Communication between your virtual machines should not be a bottleneck. Writing to IBM urging them to change their OSA-2 OCO policy is unlikely to help, but might make you feel better.
Currently, penguinvm.princeton.edu uses CTCA to talk to the VM hypervisor's TCP/IP stack, but it would also work to set up a Linux virtual machine and use it as a firewall, doing packet filtering and network address translation for the machines behind it. That machine can then talk directly to either the outside world with its OSA-2 interface or VM's stack with a different IUCV/CTCA interface.
Installation is still ugly. Basically, you're not going to get anywhere unless you understand how to install a new operating system on a System/390, and you understand how to install and set up Linux. It's a four-step process:
Build a boot loader image from the supplied files.
Boot the loader.
Dump the file system tar archive onto newly created EXT2 partitions.
Reboot and answer the configuration questions.
The first two steps require S/390 knowledge, the last two Linux knowledge. You may be able to puzzle it out from the documentation, but if you're shaky in one area or the other, it's probably a good idea to find someone who has the other base covered.
Marist College has been at the forefront of Linux/390 development. Marist's page contains the tape and disk images necessary to get started, as well as documentation, and is the place to begin. There is also an extremely active mailing list hosted at Marist, Linux-VM, which is not VM-specific, but is devoted to Linux on the mainframe, with or without VM. The file system layout of the Marist disk is a little weird if you're used to Red Hat or SuSE. This will change as actual distributions get ported.
There is already a distribution for the S/390: “Think Blue Linux” or “Iron Penguin”, based on Red Hat 6.1. At the time of this writing, it is only a downloadable collection of packages and not yet available on CD or tape.
As soon as I get the testbed P/390 I have available in Virginia fired up and networked (which should also have happened long ago by the time you read this), Richard Higson and I will begin the process of porting Debian Potato. I'm expecting it to be fairly easy, since Debian already supports quite a few platforms.
In short, by the time this is published, installation should be much simpler than it currently is. It's not insanely difficult even now, but it is comparable to Linux/x86 in early 1993, when SLS was beginning to get off the ground. It certainly is no Lizard or YAST2, and installation requires you to understand both the System/390 and Linux.