Re: [Xen-users] Sharing PCI devices

[Didier Trosset <didier_trosset@xxxxxxxxxxx>]

Petersson, Mats wrote:
> > -----Original Message-----
> > From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> > [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> > Didier Trosset
> > Sent: 23 May 2007 13:21
> > To: xen-users@xxxxxxxxxxxxxxxxxxx
> > Subject: [Xen-users] Sharing PCI devices
> >
> >
> > I'd like to know if there is a prefered way to go for sharing 
> > a PCI device 
> > between a couple of domUs. Is it better going para-virtualized, or 
> > full-virtualized ?
> It is not (currently) possible to use hardware PCI devices inside a HVM
> domain without major changes to the driver (and I'm not even sure if
> there's sufficient support in the Hypercall interface to support the
> necessary operations as it is today - but for certain, the driver needs
> to be AWARE that it's in a virtual machine rather than a regular
> "bare-metal" OS). 
>
> This is basically because Xen hides the actual memory layout of the
> guest's memory from the guest, and thus when the guest THINKS it knows
> the physical address (which is necessary to pass through to a PCI device
> for it's bus mastering operations) it will not be able to give the right
> information (but rather tell the same lies that it was told by Xen).
> This leads to completely incorrect memory accesses from the device,
> which in turn will lead to major mishaps in the device operation itself
> (e.g. it sends the wrong data out on the line, reads "garbage"
> instructions from memory, or overwrites the wrong area of memory with
> it's data [network packet, disk data or whatever it may be]). In the
> end, you'll probably end up with a VERY corrupted system that doesn't
> behave anywhere like the right way. 
This means that I could make my device working in a kind of degraded mode, 
where it would only be slave on the PCI bus (i.e. no DMA with bus mastering, 
in either direction, and no interrupts) within a HVM. Is this correct 
(leaving aside the concurrent access problems) ?
> In the future, hardware with IOMMU (I/O Device Memory mapping) will be
> able to "redirect" the "believed" memory address to the actual physical
> location known by Xen. But this will not happen very soon. 
>
> So the answer here is that you NEED to have a Para-virtual kernel, at
> least until IOMMU is available. 
>
> ---- 8< ----
--
Didier Trosset-Moreau
Agilent Technologies
Geneva, Switzerland

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