Re: kvm PCI assignment & VFIO ramblings

quoted

I wouldn't use uiommu for that.

Any particular reason besides saving a file descriptor?

We use it today, and it seems like a cleaner API than what you propose
changing it to.

Well for one, we are back to square one vs. grouping constraints.

 .../...

If we in singleton-group land were building our own "groups" which were sets
of devices sharing the IOMMU domains we wanted, I suppose we could do away
with uiommu fds, but it sounds like the current proposal would create 20
singleton groups (x86 iommu w/o PCI bridges => all devices are partitionable
endpoints).  Asking me to ioctl(inherit) them together into a blob sounds
worse than the current explicit uiommu API.

I'd rather have an API to create super-groups (groups of groups)
statically and then you can use such groups as normal groups using the
same interface. That create/management process could be done via a
simple command line utility or via sysfs banging, whatever...

Cheers,
Ben.

Thanks,
Aaron

quoted

Another option is to make that static configuration APIs via special
ioctls (or even netlink if you really like it), to change the grouping
on architectures that allow it.

Cheers.
Ben.

quoted

-Aaron

quoted

As necessary in the future, we can
define a more high performance dma mapping interface for streaming dma
via the group fd.  I expect we'll also include architecture specific
group ioctls to describe features and capabilities of the iommu.  The
group fd will need to prevent concurrent open()s to maintain a 1:1 group
to userspace process ownership model.

Also on the table is supporting non-PCI devices with vfio.  To do this,
we need to generalize the read/write/mmap and irq eventfd interfaces.
We could keep the same model of segmenting the device fd address space,
perhaps adding ioctls to define the segment offset bit position or we
could split each region into it's own fd (VFIO_GET_PCI_BAR_FD(0),
VFIO_GET_PCI_CONFIG_FD(), VFIO_GET_MMIO_FD(3)), though we're already
suffering some degree of fd bloat (group fd, device fd(s), interrupt
event fd(s), per resource fd, etc).  For interrupts we can overload
VFIO_SET_IRQ_EVENTFD to be either PCI INTx or non-PCI irq (do non-PCI
devices support MSI?).

For qemu, these changes imply we'd only support a model where we have a
1:1 group to iommu domain.  The current vfio driver could probably
become vfio-pci as we might end up with more target specific vfio
drivers for non-pci.  PCI should be able to maintain a simple -device
vfio-pci,host=bb:dd.f to enable hotplug of individual devices.  We'll
need to come up with extra options when we need to expose groups to
guest for pvdma.

Hope that captures it, feel free to jump in with corrections and
suggestions.  Thanks,

Alex