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Re: [Xen-users] Consolidating multiple machines. Feasible?

On Wednesday 14 February 2007, Daniel McAllansmith wrote:
> Given hardware based virtualisation, are devices (eg graphics, sound, tv
> cards...) in domUs going to work just like it was the native OS?

no.  'hardware based virtualisation' only means that Intel and AMD added some 
tricks to their processors to overcome some limitations that used to make CPU 
virtualisation much harder than it is on almost any other CPU.

only CPU and RAM are virtualised; the rest of the hardware (net, block device, 
screen, keyboard, PCI, USB) is all emulated, using code borrowed from qemu

you can select PCI devices to be accessed by a domU instead of by dom0; but 
it's very experimental on HVM domUs (works on PV domUs).  even with that, 
screen cards are usually the thoughest to reassign (IOW: no)

> Can individual USB devices be reserved for use by a domU?

there's some experimental code for that, but AFAIK, not ready yet.

> Can domUs be nested?  E.g. run a windows machine in the domU linux
> workstation (in a window).

not on pure Xen; but you could run Qemu on a domU.  no accelerator, 
unfortunately (that means the slowest variety of qemu)

> Can hardware be relinquished by a domU?  Perhaps through some sort of
> tricky virtual machine migration?

no.  PCI initialisation is very complex; you'd need a guest OS that knows how 
to do that and a lot of help from the hypervisor.  much easier is to use it 
from dom0 and share it like on a network.

> What happens if you try to give reserved hardware to a domU when another
> domU already has it?

you can't.  the hypervisor won't allow. (and the userland tools check for 
conflicts before starting the domU)

> It seems desirable to have as little as possible running on dom0.  Are
> there management tools which handle headless dom0 and the need to shutdown
> a certain domU to free up hardware so the desired domU can be started up?

not that i've heard.  anyway, the hardware in question would have to be 
hard-reset to return it to power-up state, so that the new guest OS would 
find it as expected.  i don't know if that's possible without interfering 
with the whole PCI bus.


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