A look at Xen
Posted Jun 24, 2005 23:34 UTC (Fri) by erich
In reply to: A look at Xen
Parent article: A look at Xen
Do you know the basics of OS programming? About the ring model?
They are mentioned in http://en.wikipedia.org/wiki/X86
Xen runs in ring-0, requiring the OS to run in the lower rings.
Usually linux itself is in ring 0; userspace apps run in ring 2 or 3.
Basically, inner rings have more privileges, and are protected from outer rings.
Xen manages the memory on a high level, and linux instead of just handling memory itself, requests memory from xen via special calls. Basically thats the same way, the linux kernel is protected from userspace applications.
If Linux would try to do it directly, it would cause a segmentation fault; but having this protection, it cannot trash xen or the other vms.
VMWare probably detects these calls in the binary and replaces that code. VMware is MUCH closer to emulating a separate PC; Xen just takes away some privileges from the OS in order to separate them, and requires the OS to "politely ask" for the resources. Thats why you need a different kernel.
But since everything accessing the hardware is handled by the linux kernel, all changes needed for Xen are within the kernel.
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