kvm PCI assignment & VFIO ramblings

[Alexander Graf]

On 26.08.2011, at 10:24, Joerg Roedel wrote:

> 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.

Yup :)

> 
>> 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).

I agree. The same it works with tun today. You can either have qemu spawn a tun device dynamically or have a preallocated one you use. If you run qemu as a user (which I always do), I preallocate a tun device and attach qemu to it.

> 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).

Agreed. As long as the kernel backend can always figure out the hw-groups, we're good :)


Alex