Re: kvm PCI assignment & VFIO ramblings

Joerg Roedel
2011-08-26T15:24:07Z

On Fri, Aug 26, 2011 at 09:07:35AM -0500, Alexander Graf wrote:
> On 26.08.2011, at 04:33, Roedel, Joerg wrote:
> >
> > The reason is that you mean the usability for the programmer and I mean
> > it for the actual user of qemu :)
>
> No, we mean the actual user of qemu. The reason being that making a
> device available for any user space application is an administrative
> task.
>
> Forget the KVM case for a moment and think of a user space device
> driver. I as a user am not root. But I as a user when having access to
> /dev/vfioX want to be able to access the device and manage it - and
> only it. The admin of that box needs to set it up properly for me to
> be able to access it.

Right, and that task is being performed by attaching the device(s) in
question to the vfio driver. The rights-management happens on the
/dev/vfio/$group file.

> So having two steps is really the correct way to go:
>
> * create VFIO group
> * use VFIO group
>
> because the two are done by completely different users. It's similar
> to how tun/tap works in Linux too. Of course nothing keeps you from
> also creating a group on the fly, but it shouldn't be the only
> interface available. The persistent setup is definitely more useful.

I see the use-case. But to make it as easy as possible for the end-user
we can do both.

So the user of (qemu again) does this:

# vfio-ctl attach 00:01.0
vfio-ctl: attached to group 8
# vfio-ctl attach 00:02.0
vfio-ctl: attached to group 16
$ qemu -device vfio-pci,host=00:01.0 -device vfio,host=00:01.0 ...

which should cover the usecase you prefer. Qemu still creates the
meta-group that allow the devices to share the same page-table. But what
should also be possible is:

# qemu -device vfio-pci,host=00:01.0 -device vfio-pci,host=00:02.0

In that case qemu detects that the devices are not yet bound to vfio and
will do so and also unbinds them afterwards (essentially the developer
use-case).

Your interface which requires pre-binding of devices into one group by
the administrator only makes sense if you want to force userspace to
use certain devices (which do not belong to the same hw-group) only
together. But I don't see a usecase for defining such constraints (yet).

Joerg