Re: [RFC] /dev/ioasid uAPI proposal

[David Gibson]

On Tue, Jun 01, 2021 at 02:56:43PM -0300, Jason Gunthorpe wrote:
> On Tue, Jun 01, 2021 at 08:38:00AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > > Sent: Saturday, May 29, 2021 3:59 AM
> > > 
> > > On Thu, May 27, 2021 at 07:58:12AM +0000, Tian, Kevin wrote:
> > > >
> > > > 5. Use Cases and Flows
> > > >
> > > > Here assume VFIO will support a new model where every bound device
> > > > is explicitly listed under /dev/vfio thus a device fd can be acquired w/o
> > > > going through legacy container/group interface. For illustration purpose
> > > > those devices are just called dev[1...N]:
> > > >
> > > > 	device_fd[1...N] = open("/dev/vfio/devices/dev[1...N]", mode);
> > > >
> > > > As explained earlier, one IOASID fd is sufficient for all intended use cases:
> > > >
> > > > 	ioasid_fd = open("/dev/ioasid", mode);
> > > >
> > > > For simplicity below examples are all made for the virtualization story.
> > > > They are representative and could be easily adapted to a non-virtualization
> > > > scenario.
> > > 
> > > For others, I don't think this is *strictly* necessary, we can
> > > probably still get to the device_fd using the group_fd and fit in
> > > /dev/ioasid. It does make the rest of this more readable though.
> > 
> > Jason, want to confirm here. Per earlier discussion we remain an
> > impression that you want VFIO to be a pure device driver thus
> > container/group are used only for legacy application.
> 
> Let me call this a "nice wish".
> 
> If you get to a point where you hard need this, then identify the hard
> requirement and let's do it, but I wouldn't bloat this already large
> project unnecessarily.
> 
> Similarly I wouldn't depend on the group fd existing in this design
> so it could be changed later.

I don't think presence or absence of a group fd makes a lot of
difference to this design.  Having a group fd just means we attach
groups to the ioasid instead of individual devices, and we no longer
need the bookkeeping of "partial" devices.

> > From this comment are you suggesting that VFIO can still keep
> > container/ group concepts and user just deprecates the use of vfio
> > iommu uAPI (e.g. VFIO_SET_IOMMU) by using /dev/ioasid (which has a
> > simple policy that an IOASID will reject cmd if partially-attached
> > group exists)?
> 
> I would say no on the container. /dev/ioasid == the container, having
> two competing objects at once in a single process is just a mess.

Right.  I'd assume that for compatibility, creating a container would
create a single IOASID under the hood with a compatiblity layer
translating the container operations to iosaid operations.

> If the group fd can be kept requires charting a path through the
> ioctls where the container is not used and /dev/ioasid is sub'd in
> using the same device FD specific IOCTLs you show here.

Again, I don't think it makes much difference.  The model doesn't
really change even if you allow both ATTACH_GROUP and ATTACH_DEVICE on
the IOASID.  Basically ATTACH_GROUP would just be equivalent to
attaching all the constituent devices.

> I didn't try to chart this out carefully.
> 
> Also, ultimately, something need to be done about compatability with
> the vfio container fd. It looks clear enough to me that the the VFIO
> container FD is just a single IOASID using a special ioctl interface
> so it would be quite rasonable to harmonize these somehow.
> 
> But that is too complicated and far out for me at least to guess on at
> this point..
> 
> > > Still a little unsure why the vPASID is here not on the gva_ioasid. Is
> > > there any scenario where we want different vpasid's for the same
> > > IOASID? I guess it is OK like this. Hum.
> > 
> > Yes, it's completely sane that the guest links a I/O page table to 
> > different vpasids on dev1 and dev2. The IOMMU doesn't mandate
> > that when multiple devices share an I/O page table they must use
> > the same PASID#. 
> 
> Ok..
> 
> 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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