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Re: [Xen-devel] Paravirtualised drivers for fully virtualised domains

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Subject: Re: [Xen-devel] Paravirtualised drivers for fully virtualised domains
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Tue, 18 Jul 2006 17:23:31 +0100
Cc: Steve Ofsthun <sofsthun@xxxxxxxxxxxxxxx>, sos22@xxxxxxxxx
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> > These drivers are explicitly not considered by XenSource to be an
> > alternative to improving the performance of the ioemu devices.
> > Rather, work on both will continue in parallel.
> I agree.  Both activities are worth developing.

There's lots of stuff still to be done to make the ioemu devices work better, 
even if some users wish to use PV drivers directly some will still want the 
simplicity of working "out of the box".

> Actually presenting two copies of the same device to linux can cause
> its own problems.  Mounting using LABEL= will complain about duplicate
> labels.  However, using the device names directly seems to work.  With
> this approach it is possible to decide in the guest whether to mount
> a device as an emulated disk or a PV disk.

We should *really* have interlocks in dom0 to prevent a guest from accessing 
both simultaneously :-)

Initially, we could just allow the user only to configure as either model, not 
both (using a check in Xend, as we do for checking mounted partitions, etc).  
To support what you propose we'd probably have to add a little control plane 
stuff, but I think it'd be worth it to avoid too many people damaging stuff!

To mangle a quote I once saw online: duplicate device access can be used to 
hunt both foot and game, but only one will feed your family.


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