Re: [Qemu-devel] [RFC] Device isolation infrastructure v2

[David Gibson <david@gibson.dropbear.id.au>]

On Mon, Dec 19, 2011 at 10:56:40PM +0000, David Woodhouse wrote:
> On Tue, 2011-12-20 at 09:31 +1100, David Gibson wrote:
> > When we're running paravirtualized under pHyp, it's impossible to
> > merge multiple PEs into one domain per se.  We could fake it rather
> > nastily by replicating all map/unmaps across mutiple PEs.  When
> > running bare metal, we could do so a bit more nicely by assigning
> > multiple PEs the same TCE pointer, but we have no mechanism to do so
> > at present. 
> 
> VT-d does share the page tables, as you could on bare metal. But it's an
> implementation detail — there's nothing *fundamentally* wrong with
> having to do the map/unmap for each PE, is there? It's only at VM setup
> time, so it doesn't really matter if it's slow.
> 
> Surely that's the only way you're going to present the guest with the
> illusion of having no IOMMU; so that DMA to any given guest physical
> address "just works".
> 
> On the other hand, perhaps you don't want to do that at all. Perhaps
> you're better off presenting a virtualised IOMMU to the guest and
> *insisting* that it fully uses it in order to do any DMA at all?

Not only do we want to, we more or less *have* to.  Existing kernels,
which are used to being paravirt under phyp expect and need a paravirt
iommu.  DMA without iommu setup just doesn't happen.  And the
map/unmap hypercalls are frequently a hot path, so slow does matter.

The other problem is that each domain's IOVA window is often fairly
small, a limitation that would get even worse if we try to put too
many devices in there.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson