Re: [RFC] /dev/ioasid uAPI proposal

Jason Gunthorpe
2021-06-04T12:31:11Z

On Fri, Jun 04, 2021 at 12:44:28PM +0200, Enrico Weigelt, metux IT consult wrote:
> On 02.06.21 19:24, Jason Gunthorpe wrote:
>
> Hi,
>
> >> If I understand this correctly, /dev/ioasid is a kind of "common
> supplier"
> >> to other APIs / devices. Why can't the fd be acquired by the
> >> consumer APIs (eg. kvm, vfio, etc) ?
> >
> > /dev/ioasid would be similar to /dev/vfio, and everything already
> > deals with exposing /dev/vfio and /dev/vfio/N together
> >
> > I don't see it as a problem, just more work.
>
> One of the problems I'm seeing is in container environments: when
> passing in an vfio device, we now also need to pass in /dev/ioasid,
> thus increasing the complexity in container setup (or orchestration).

Containers already needed to do this today. Container orchestration is
hard.

> And in such scenarios you usually want to pass in one specific device,
> not all of the same class, and usually orchestration shall pick the
> next free one.
>
> Can we make sure that a process having full access to /dev/ioasid
> while only supposed to have to specific consumer devices, can't do
> any harm (eg. influencing other containers that might use a different
> consumer device) ?

Yes, /dev/ioasid shouldn't do anything unless you have a device to
connect it with. In this way it is probably safe to stuff it into
every container.

> > Having FDs spawn other FDs is pretty ugly, it defeats the "everything
> > is a file" model of UNIX.
>
> Unfortunately, this is already defeated in many other places :(
> (I'd even claim that ioctls already break it :p)

I think you are reaching a bit :)

> It seems your approach also breaks this, since we now need to open two
> files in order to talk to one device.

It is two devices, thus two files.

Jason