Re: RFC: Device isolation infrastructure

[David Gibson <dwg@au1.ibm.com>]

On Wed, Dec 07, 2011 at 11:23:10PM -0700, Alex Williamson wrote:
> 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.

No, I was assuming that "permission to detach" could be handed out to
a user before this step.  uid/gid/mode attributes in sysfs would
suffice, though there might be better ways.

-- 
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_!
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