Re: [Xen-devel] a question about xen in HVM

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

>   I have a question about HVM and PV, in the HVM,xen is running in the
> root mode,and should vmentry or vmexit to HVM vm,and in the PV,xen is
> running in the ring0, am i right?
>   and if some PV vm and some HVM vm are running on the physical node
> with VTsupport,then which mode does the xen run in? in ring0 or what
> about it ?

On a machine which is not VT capable, there is no "root" or "non-root" mode, 
so Xen runs in ring 0 and guests ring in rings 1-3.

On a machine which is VT capable you have "root mode" and "non-root 
mode".  "root mode" is basically the same as the normal operation of a non-VT 
processor; it still has the rings 0-3, which fulfill the same purpose as they 
did on non-VT machines.  They still work the same.  "non-root mode" is a 
second set of CPU protection rings 0-3, which Xen uses for HVM guests and 
which operate independently of those rings in "root mode".

If you're running PV and HVM guests at once then you have:

* Xen runs in ring 0 of "root mode"
* dom0 runs in ring 1-3 of "root mode"
* PV domUs run in ring 1-3 of "root mode"
* HVM domUs run in rings 0-3 of "non-root mode"

Note that when VT is available, "root mode" and "non-root" mode both have a 
set of their own rings.  So it's not a question of whether something runs in 
ring "n" anymore, it's a question of whether it runs in ring n /of root mode/ 
or ring n /of non-root mode/.

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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