Re: RFC: Device isolation infrastructure

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 2011-12-08 at 13:43 +1100, David Gibson wrote:
> On Wed, Dec 07, 2011 at 12:45:20PM -0700, Alex Williamson wrote:
> > So the next problem is that while the group is the minimum granularity
> > for the iommu, it's not necessarily the desired granularity.  iommus
> > like VT-d have per PCI BDF context entries that can point to shared page
> > tables.  On such systems we also typically have singleton isolation
> > groups, so when multiple devices are used by a single user, we have a
> > lot of duplication in time and space.  VFIO handles this by allowing
> > groups to be "merged".  When this happens, the merged groups point to
> > the same iommu context.  I'm not sure what the plan is with isolation
> > groups, but we need some way to reduce that overhead.
> 
> Right.  So, again, I intend that mutiple groups can go into one
> domain.  Not entirely sure of the interface yet.  One I had in mind
> was to borrow the vfio1 interface, so you open a /dev/vfio (each open
> gives a new instance).  Then you do an "addgroup" ioctl which adds a
> group to the domain.  You can do that multiple times, then start using
> the domain.

This also revisits one of the primary problems of vfio1, the dependency
on a privileged uiommu domain creation interface.  Assigning a user
ownership of a group should be a privileged operation.  If a privileged
user needs to open /dev/vfio, add groups, then drop privileges and hand
the open file descriptor to an unprivileged user, the interface becomes
much harder to use.  "Hot merging" becomes impossible.

Alex