Re: Plan for /dev/ioasid RFC v2

[David Gibson]

On Thu, Jun 17, 2021 at 08:10:04PM -0300, Jason Gunthorpe wrote:
> 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..

I don't know that there's a "definitive" reason.  My concern (and I
think Alex's as well) is that if there's no (3), it tends to lead to a
lack of (2), and lack of (2) tends to make people sloppily forget
about (1) and lead to breakage.

> > > 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
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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