Re: [Xen-devel] Xen virtual IOMMU high level design doc

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Stefano Stabellini [mailto:sstabellini@xxxxxxxxxx]
> Sent: Thursday, November 24, 2016 3:09 AM
> 
> On Wed, 23 Nov 2016, Edgar E. Iglesias wrote:
> > On Wed, Aug 17, 2016 at 08:05:51PM +0800, Lan, Tianyu wrote:
> > > Hi All:
> > >      The following is our Xen vIOMMU high level design for detail
> > > discussion. Please have a look. Very appreciate for your comments.
> > > This design doesn't cover changes when root port is moved to hypervisor.
> > > We may design it later.
> >
> > Hi,
> >
> > I have a few questions.
> >
> > If I understand correctly, you'll be emulating an Intel IOMMU in Xen.
> > So guests will essentially create intel iommu style page-tables.
> >
> > If we were to use this on Xen/ARM, we would likely be modelling an ARM
> > SMMU as a vIOMMU. Since Xen on ARM does not use QEMU for emulation, the
> > hypervisor OPs for QEMUs xen dummy IOMMU queries would not really be used.
> > Do I understand this correctly?
> 
> I think they could be called from the toolstack. This is why I was
> saying in the other thread that the hypercalls should be general enough
> that QEMU is not the only caller.
> 
> For PVH and ARM guests, the toolstack should be able to setup the vIOMMU
> on behalf of the guest without QEMU intervention.
> 
> 
> > Has a platform agnostic PV-IOMMU been considered to support 2-stage
> > translation (i.e VFIO in the guest)? Perhaps that would hurt map/unmap
> > performance too much?
> 
> That's an interesting idea. I don't know if that's feasible, but if it
> is not, then we need to be able to specify the PV-IOMMU type in the
> hypercalls, so that you would get Intel IOMMU on x86 and SMMU on ARM.
> 
> 

Not considered yet. PV is always possible as we've done for other I/O
devices. Ideally it could be designed being more efficient than full
emulation of vendor specific IOMMU, but also means requirement of
maintaining a new guest IOMMU driver and limitation of supporting
only newer version guest OSes. It's a tradeoff... at least not compelling 
now (may consider when we see a real need in the future).

Thanks
Kevin

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