Re: kvm PCI assignment & VFIO ramblings

On Thu, 2011-08-04 at 12:41 +0200, Joerg Roedel wrote:

On Mon, Aug 01, 2011 at 02:27:36PM -0600, Alex Williamson wrote:

quoted

It's not clear to me how we could skip it.  With VT-d, we'd have to
implement an emulated interrupt remapper and hope that the guest picks
unused indexes in the host interrupt remapping table before it could do
anything useful with direct access to the MSI-X table.  Maybe AMD IOMMU
makes this easier?

AMD IOMMU provides remapping tables per-device, and not a global one.
But that does not make direct guest-access to the MSI-X table safe. The
table contains the table contains the interrupt-type and the vector
which is used as an index into the remapping table by the IOMMU. So when
the guest writes into its MSI-X table the remapping-table in the host
needs to be updated too.

Right, you need paravirt to avoid filtering :-)

IE the problem is two fold:

 - Getting the right value in the table / remapper so things work
(paravirt)

 - Protecting against the guest somewhat managing to change the value in
the table (either directly or via a backdoor access to its own config
space).

The later for us comes from the HW PE filtering of the MSI transactions.

Cheers,
Ben.