Re: Plan for /dev/ioasid RFC v2

[Jason Gunthorpe]

On Thu, Jun 17, 2021 at 02:45:46PM +1000, David Gibson wrote:
> On Wed, Jun 09, 2021 at 09:39:19AM -0300, Jason Gunthorpe wrote:
> > On Wed, Jun 09, 2021 at 02:24:03PM +0200, Joerg Roedel wrote:
> > > On Mon, Jun 07, 2021 at 02:58:18AM +0000, Tian, Kevin wrote:
> > > > -   Device-centric (Jason) vs. group-centric (David) uAPI. David is not fully
> > > >     convinced yet. Based on discussion v2 will continue to have ioasid uAPI
> > > >     being device-centric (but it's fine for vfio to be group-centric). A new
> > > >     section will be added to elaborate this part;
> > > 
> > > I would vote for group-centric here. Or do the reasons for which VFIO is
> > > group-centric not apply to IOASID? If so, why?
> > 
> > VFIO being group centric has made it very ugly/difficult to inject
> > device driver specific knowledge into the scheme.
> > 
> > The device driver is the only thing that knows to ask:
> >  - I need a SW table for this ioasid because I am like a mdev
> >  - I will issue TLPs with PASID
> >  - I need a IOASID linked to a PASID
> >  - I am a devices that uses ENQCMD and vPASID
> >  - etc in future
> 
> mdev drivers might know these, but shim drivers, like basic vfio-pci
> often won't.

The generic drivers say 'I will do every kind of DMA possible', which
is in-of-itself a special kind of information to convey.

There are alot of weird corners to think about here, like what if the
guest asks for a PASID on a mdev that doesn't support PASID, but
hooked to a RID that does or other quite nonsense combinations. These
need to be blocked/handled/whatever properly, which is made much
easier if the common code actually knows detail about what is going
on.

> I still think you're having a tendency to partially conflate several
> meanings of "group":
> 	1. the unavoidable hardware unit of non-isolation
> 	2. the kernel internal concept and interface to it
> 	3. the user visible fd and interface

I think I have those pretty clearly seperated :)
 
> We can't avoid having (1) somewhere, (3) and to a lesser extent (2)
> are what you object to.

I don't like (3) either, and am yet to hear a definitive reason why we
must have it..
 
> > The current approach has the group try to guess the device driver
> > intention in the vfio type 1 code.
> 
> I agree this has gotten ugly.  What I'm not yet convinced of is that
> reworking groups to make this not-ugly necessarily requires totally
> minimizing the importance of groups.

I think it does - we can't have the group in the middle and still put
the driver in chrage, it doesn't really work.

At least if someone can see an arrangement otherwise lets hear it -
start with how to keep groups and remove the mdev hackery from type1..

Jason