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OLS: Xen and UML

Posted Jul 25, 2005 15:45 UTC (Mon) by GreyWizard (guest, #1026)
Parent article: OLS: Xen and UML

"Despite the fact that UML now runs in ring 0, it is still a user-mode process, and thus still deserves its name."

Forgive my ignorance but how is this possible? I had always understood ring 0 to be kernel mode and some other ring (normally ring 3 on x86) to be user mode. Can UML have its cake and eat it too?

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User-mode in ring 0

Posted Jul 25, 2005 15:50 UTC (Mon) by corbet (editor, #1) [Link]

The point is that it's not a real ring 0, it's a virtualized one created by the hardware. It's real in that it is protected from the virtualized ring 3 where UML's user space runs, and it can do a few semi-privileged things, but it's not the same ring 0 that the host system runs in.