RE: [Xen-users] Providing a Domain with a Device

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of John Smith
> Sent: 22 October 2006 16:20
> To: Mathew Brown
> Cc: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] Providing a Domain with a Device
> 
> Mathew Brown wrote:
> > Hi,
> >   I'd read on the Xen FAQ (http://wiki.xensource.com/xenwiki/XenFaq)
> >   that Xen can provide certain devices (PCI devices for example), to
> >   domains.  Does this mean that I can provide my nVidia 
> Graphics card to
> >   a domU or would I only be able to use it in dom0?  Does 
> it matter if
> >   I'm using HVM or a para-virtualized domain?  Thanks for your help.
> 
> Hi Mathew,
> 
>       I tried this last week and was a bit dissapointed when it turned
> out to be a case of "either/or". I made my graphics card 
> available in a
> domU with as a result that I lost my dom0 console.
> 
>       I guess you can work around this by redirecting your 
> dom0 console
> to serial or network, but it isn't the ideal solution, which 
> would be to
> make the device shared by both. Maybe something for the future.

Unfortunately, sharing graphics requires extra software somewhere, and
the easiest[1] option for this is a para-virtual driver for the DomU. 

As Muli mentioned, for HVM, it's necessary to have an IOMMU to allow the
graphics card to be "fooled" into thinking that the guest's memory map
is the true memory map.

[1] Easiest doesn't mean easy! Graphics drivers are VERY complex beasts
- I worked for a graphics card company a while back, and the driver is
tens of megabytes of source-code (roughly the size of the i386 Linux
kernel with all the drivers and common-source code, but not as large as
ALL the different CPU-flavours that Linux supports). Much of that can
probably be "punted" (a Windows term for "let Windows do it through
simpler functions"), so in theory you could probably build a small
driver that only does simple things - but it would then be slower than
if you support direct functionality... 

If you want a more difficult method of sharing graphics, try writing a
cross-domain (with multi-OS support!) cooperative driver - I'll come
visit at the mental institution, ok? ;-)

--
Mats

> 
> Sincerely,
> 
> Jan.
> 
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