Re: kvm PCI assignment & VFIO ramblings

On Fri, Aug 05, 2011 at 08:26:11PM +1000, Benjamin Herrenschmidt wrote:

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

quoted

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 :-)

Or a shadow MSI-X table like done on x86. How to handle this seems to be
platform specific. As you indicate there is a standardized paravirt
interface for that on Power.

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.

Right. The second part of the problem can be avoided with
interrupt-remapping/filtering hardware in the IOMMUs.

	Joerg