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


> -----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


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