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[Xen-users] Re: PCI Passthrough to VMX Guest

  • To: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
  • From: "David Goodlad" <dgoodlad@xxxxxxxxx>
  • Date: Tue, 21 Mar 2006 09:01:51 -0800
  • Cc: mark.williamson@xxxxxxxxxxxx, xen-users@xxxxxxxxxxxxxxxxxxx
  • Delivery-date: Tue, 21 Mar 2006 17:03:21 +0000
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On 3/21/06, Petersson, Mats <Mats.Petersson@xxxxxxx> wrote:
> > -----Original Message-----
> > From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
> > [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of
> > M.A. Williamson
> > Sent: 21 March 2006 15:46
> > To: David Goodlad
> > Cc: xen-users@xxxxxxxxxxxxxxxxxxx
> > Subject: Re: [Xen-users] PCI Passthrough to VMX Guest
> >
> > Sorry, PCI passthrough to VMX guests isn't supported - the
> > guest has to be paravirt and be running the PCI frontend driver.
> >
> > The VMX spec does include some helpful features for passing
> > PCI devices through to a guest, and I'm sure it's possible to
> > make it work - in principle. I don't know of anyone actually
> > planning to hack on it though :-(
> One of the problems with this is that the OS/Driver that supports the
> nVidia (or other graphics adapter) will need to actually know it's
> physical addresses in memory - something that it doesn't, because the
> HVM solution may well tell the OS that it's got 512MB of memory from 0
> to 512M, but it's ACTUALLY living at 512M to 1G. So when the graphics
> driver says "You have a bitmap at 128MB", it should actually say "You
> have a bitmap at 640MB". Until there's an IOMMU implementation, there's
> nothing we can do about this.
> So even if you COULD assign your PCI device to the DomU, it still
> wouldn't do the right thing... :-(
> So until then, there's a bit of a problem implementing any complex
> hardware support in a virtual machine. There may be ways to solve this,
> but they are non-trivial (and most like specific to the particular
> hardware...).
> --
> Mats

Ahh, that clears things up.  I never thought of the memory addressing
issue!  I assume that this issue is handled in paravirtualized
machines by the PCI frontend driver?


Dave Goodlad
dgoodlad@xxxxxxxxx or dave@xxxxxxxxxx

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