editor forgot UML (again) [LWN.net]

editor forgot UML (again)

Posted May 4, 2006 21:21 UTC (Thu) by proski (subscriber, #104)
In reply to: editor forgot UML (again) by amtota
Parent article: Xen and the new processors

Try running "unmodified" Windows that you built with ARCH=um :-)

The whole point of full virtualization is that you run unmodified binaries, whether it's Linux, QNX, OS/2 or CP/M. I heard that Linux compiled to run in Xen would run faster than Linux built to run on raw hardware. So you really care about full virtualizaion if you cannot or don't want to recompile anything.

It's amazing that it took so many years to support IA-32 virtualization. The 16-bit virtual machine support appeared in Intel 80386 in 1986 - twenty years ago.

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Linux does require modification

Posted May 5, 2006 0:36 UTC (Fri) by giraffedata (subscriber, #1954) [Link]

The whole point of full virtualization is that you run unmodified binaries

It's simpler than that. Even at the source code level, Linux must be modified to run under Linux. The reason you can "just build with ARCH=um" is that someone has made those modifications.

When people speak of the value of a virtualization that lets you run unmodified code, they're concerned about all that code that hasn't been modified yet for that particular kind of virtual machine, and probably never will be. E.g. Windows. If there's only one program you want to run in the virtual machine (e.g. Linux), and there's already a modified version available, you can cross that concern off your list and consider UML, XEN, etc.

By the way, if you don't require a virtual machine that exactly emulates some existing real machine, then the universe of virtualization is much larger than most people see. Any Unix process is a virtual machine. Used to be, you could run only one program on a computer at a time. The invention of operating systems like Unix let you have essentially (virtually) multiple computers based on a single piece of hardware, as long as you were willing to modify your programs for a different environment.