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

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Subject: Re: [Xen-users] Consolidating multiple machines. Feasible?
From: Daniel McAllansmith <dm.maillists@xxxxxxxxx>
Date: Mon, 19 Feb 2007 13:00:41 +1300
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Hello, Javier.

Thanks for the response.

On Friday 16 February 2007 02:05, Javier Guerra wrote:
> 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)

As far as I understand things AMDs Pacifica virtualisation added an IOMMU 
which can translate DMA transfers for HyperTransport or PCI devices.

I thought that use of the IOMMU would allow unmodified drivers in a unmodified 
domU OS access to the device (where the device has been hidden from domO).
This should allow 3D acceleration in a domU windows OS, just as can be 
achieved now using a paravirtualisation-aware driver in a domU linux.

Am I misunderstanding the purpose of the IOMMU, or does Xen not support it 

> > Can individual USB devices be reserved for use by a domU?
> there's some experimental code for that, but AFAIK, not ready yet.

From further reading I see that xen 2 supported passthrough of usb devices but 
this has bitrotted.

I see some attempts to allocate an entire separate physical USB controller to 
different domO/domU(s) but that has been unstable.  Is this a known problem?

> > 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)

So this is just the same setup on domU as running Qemu on a non-xen machine?

> > 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.
> > 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.

Would hot-pluggable devices be a solution to the initialisation and reset 

It sounds as if a domU shutting down does not reset the state of any PCI 
devices it was using, so those devices are unusable until the whole machine 
is restarted.  Is that correct?

Is it necessary for the memory allocated to domUs to be locked in?  Does it 
make a difference if para- or hardware- virtualisation is used?


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