RE: [Xen-users] HVM questions

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

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Gregory Gee
> Sent: 11 July 2006 02:22
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: RE: [Xen-users] HVM questions
> 
> 
> >> - I heard that PCI passthrough or what ever it is called  is not
> >> supported in HVM. Is that a permanent limitation or will it
> >> soon become
> >> available?  Unfortunately, I need 1 Windows VM that can 
> use dedicated
> >> PCI card.  Since Windows is only supported in HVM, then 
> this is why I
> >> ask this question.
> >
> > Yes, PCI passthrough is currently only supported for Para-virtual
> > guests.
> > It is expected to be solved (there's no technical 
> limitation for this,
> > just that someone has to write some code to support it, 
> basically - and
> > the principle is not difficult - just fill in the PCI-table 
> in the guest
> > so that the device looks like its there). There are some other
> > limitations tho'. If the PCI-card uses direct memory 
> access, you'd need
> > to use a para-virtual driver (i.e a driver that is aware of the fact
> > that physical memory isn't where the guest-OS thinks it is).
> >
> > PIO-type card will work fine without special drivers - 
> unfortunately,
> > very few PCI-cards are PIO-based...
> > --
> > Mats
> 
>   Interesting.  Mixed answer.  So it sounds like it will be 
> supported, but
> for specific PCI cards only or a para-vitual driver(SOL if guest in
> Windows I guess?).  Do vendors usually list if a card is PIO?

Probably not - but reading the specs or looking at the driver would tell
(to someone who understands how these things work). PIO would be used
for cards that handle very little data at a time, serial cards or simple
cards for intefacing with external sensors for example. Anything that
does high-speed/mass-transfer work, like grapics, network- and
disk-controllers would definitely use DMA. 

In the future, machines will have a IOMMU-device so the hypervisor
should be able to set up a mapping of what the guest thinks is physical
addresses and what the REAL physical address is, so that PCI-cards can
be used without modifications to the driver. 

--
Mats
> 
> Thanks,
> Greg
> 
> 
> 
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