S/390: The Linux Dream Machine - page 7
Linux Everywhere: More than a Slogan
Naturally, you'll want to have TCP/IP networking services running right away. The standard boot images from Marist College support the IBM "OSA" network adapter hardware, which is in common use, but for now each Linux image running needs to have its own dedicated OSA port (fortunately, this is a multiport board, so that's usually possible). Several methods are being discussed as to how best to share the OSA adapter hardware across multiple Linux images.
Connecting one Linux image to another within the system itself is another matter entirely. Here IBM provides a low-level interface called the Inter User Communications Vehicle (IUCV), which allows memory-mapped messaging between virtual machines by transferring data from one address space to another. The IUCV in Linux can be mapped to a set of PPP-like (point-to-point) adapters called iucv0, iucv1, and so on. These can be given IP addresses and then used like any other point-to-point network connections. At the present time, the "other end" of this link must be tied to a specified virtual machine, which implies that one would probably dedicate a single virtual Linux machine with multiple IUCV links to serve as a router for all the other virtual Linux machines, thus saving physical network ports.
Some have discussed the possibility of making a virtual network adapter as a wrapper around IUCV, which would mean that all Linux instances on a single physical mainframe would communicate over an internal "virtual LAN" at gigabit speeds. Remember those multitiered client/server applications, or the ISP application, that I mentioned earlier? The IUCV virtual LAN would be an enormous benefit to such an installation.
By the way, there are some nice security implications to running multiple Linux instances on a single VM host. Even being root on Linux doesn't take you beyond being a normal user on the underlying host. So an intruder who cracks one of the Linux virtual machines doesn't automatically get access to others, or to the host operating system, unless the system administrator was nï¿½ive enough to use the same password everywhere. Systems on the Internet are often split across multiple machines to provide this level of isolation, but Linux under VM on an S/390 can do it all in one box.
Support and Contributing to Development
There is an extremely active Internet community surrounding the Linux on S/390 ports (plural), and the Marist College e-mail list is a central part of that community. Most of the development effort is focused on IBM's port right now, but Linas Vepstas is a "regular" on the Marist College list and a good deal of the discussion applies to either port. There are some extremely smart people, and some damned fine hackers, on that e-mail list. My degree is in Computer Engineering, and I thought I knew hardware pretty well until I started chatting with these folks. There are some IBM engineers from Germany and the United States as well as some third-party mainframe administrators with long years of experience in the field. Yet I have found these folks to be exceedingly patient with new people, myself included, as we each try to understand whichever end of this two-worlds bridge we didn't yet know.
Contributions to the development effort are most welcome, and this project is still young enough to have lots of work remaining. There is an active effort underway to get as many packages ported as possible, and to post the working-and-tested binaries and source (patched if needed) on a web site. I am unfortunately not mainframe-literate enough to make any real contribution to the code itself, but writing this article to bring more awareness to the project is my own way of trying to "give something back."
There is a fair-to-middling chance that IBM will eventually make their Linux for S/390 port an officially supported product, but of course I can't speak for IBM and I don't have any inside tips on that. Right now it's strictly volunteer, but the volunteers are doing fabulously.
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