Re: [PATCH] VT-d: fix PCI device detach from virtual machine

[David Woodhouse <dwmw2@infradead.org>]

On Thu, 2010-06-17 at 11:35 +0800, Weidong Han wrote:
> David Woodhouse wrote:
> > So why do we bother setting up a context in the IOMMU for the device
> > itself, when no DMA will ever appear to come from this device? And
> >   
> if the device is behind PCI Express-to-PCI/PCI-X bridge, the source-id 
> may be the device bdf or the source-id provided by the bridge. so it
> needs to map the device itself.

Ah, that makes some sense, and matches what Jörg said about the AMD
IOMMU spec. Thanks.

> > likewise why do we bother setting up a context for intermediate PCI
> > bridges?
> >   
> I'm not sure if the intermediate PCI bridges are necessary. need to 
> check PCI spec.

FWIW, the AMD IOMMU doesn't do this; it only sets up the mapping for the
original device and for its 'proxy'.

> > Why not just jump straight to the 'DMA proxy' device, and use that
> > _only_?
> >   
> What's the 'DMA proxy' device? is it the upstream pcie-to-pci bridge?

Yes. Or, in the case of a certain buggy Ricoh multi-function device, it
is function zero -- all other functions do their DMA as if it came from
function zero.

This is why I'm looking at whether we need the whole tree-walking thing,
or whether we can just have a single 'proxy' device (or 'alias' as it's
called in the AMD code).

> all the device behind a pcie-to-pci bridge must be co-assigned to a 
> single domain. So it also require users to detach them together.

Do we even _have_ an API for KVM to assign multiple devices at the same
time? Or an API for KVM to _determine_ which devices are behind the same
'proxy'?

We could even have a new device hotplugged after the assignment has been
done.

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@intel.com                              Intel Corporation