kvm PCI assignment & VFIO ramblings

[Alex Williamson]

On Mon, 2011-08-22 at 19:25 +0200, Joerg Roedel wrote:
> On Sat, Aug 20, 2011 at 12:51:39PM -0400, Alex Williamson wrote:
> > We had an extremely productive VFIO BoF on Monday.  Here's my attempt to
> > capture the plan that I think we agreed to:
> > 
> > We need to address both the description and enforcement of device
> > groups.  Groups are formed any time the iommu does not have resolution
> > between a set of devices.  On x86, this typically happens when a
> > PCI-to-PCI bridge exists between the set of devices and the iommu.  For
> > Power, partitionable endpoints define a group.  Grouping information
> > needs to be exposed for both userspace and kernel internal usage.  This
> > will be a sysfs attribute setup by the iommu drivers.  Perhaps:
> > 
> > # cat /sys/devices/pci0000:00/0000:00:19.0/iommu_group
> > 42
> 
> Right, that is mainly for libvirt to provide that information to the
> user in a meaningful way. So userspace is aware that other devices might
> not work anymore when it assigns one to a guest.
> 
> > 
> > (I use a PCI example here, but attribute should not be PCI specific)
> > 
> > From there we have a few options.  In the BoF we discussed a model where
> > binding a device to vfio creates a /dev/vfio$GROUP character device
> > file.  This "group" fd provides provides dma mapping ioctls as well as
> > ioctls to enumerate and return a "device" fd for each attached member of
> > the group (similar to KVM_CREATE_VCPU).  We enforce grouping by
> > returning an error on open() of the group fd if there are members of the
> > group not bound to the vfio driver.  Each device fd would then support a
> > similar set of ioctls and mapping (mmio/pio/config) interface as current
> > vfio, except for the obvious domain and dma ioctls superseded by the
> > group fd.
> > 
> > Another valid model might be that /dev/vfio/$GROUP is created for all
> > groups when the vfio module is loaded.  The group fd would allow open()
> > and some set of iommu querying and device enumeration ioctls, but would
> > error on dma mapping and retrieving device fds until all of the group
> > devices are bound to the vfio driver.
> 
> I am in favour of /dev/vfio/$GROUP. If multiple devices should be
> assigned to a guest, there can also be an ioctl to bind a group to an
> address-space of another group (certainly needs some care to not allow
> that both groups belong to different processes).

That's an interesting idea.  Maybe an interface similar to the current
uiommu interface, where you open() the 2nd group fd and pass the fd via
ioctl to the primary group.  IOMMUs that don't support this would fail
the attach device callback, which would fail the ioctl to bind them.  It
will need to be designed so any group can be removed from the super-set
and the remaining group(s) still works.  This feels like something that
can be added after we get an initial implementation.

> Btw, a problem we havn't talked about yet entirely is
> driver-deassignment. User space can decide to de-assign the device from
> vfio while a fd is open on it. With PCI there is no way to let this fail
> (the .release function returns void last time i checked). Is this a
> problem, and yes, how we handle that?

The current vfio has the same problem, we can't unbind a device from
vfio while it's attached to a guest.  I think we'd use the same solution
too; send out a netlink packet for a device removal and have the .remove
call sleep on a wait_event(, refcnt == 0).  We could also set a timeout
and SIGBUS the PIDs holding the device if they don't return it
willingly.  Thanks,

Alex