Re: [RFC] /dev/ioasid uAPI proposal

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Jun 01, 2021 at 07:01:57AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Saturday, May 29, 2021 4:03 AM
> > 
> > On Thu, May 27, 2021 at 07:58:12AM +0000, Tian, Kevin wrote:
> > > /dev/ioasid provides an unified interface for managing I/O page tables for
> > > devices assigned to userspace. Device passthrough frameworks (VFIO,
> > vDPA,
> > > etc.) are expected to use this interface instead of creating their own logic to
> > > isolate untrusted device DMAs initiated by userspace.
> > 
> > It is very long, but I think this has turned out quite well. It
> > certainly matches the basic sketch I had in my head when we were
> > talking about how to create vDPA devices a few years ago.
> > 
> > When you get down to the operations they all seem pretty common sense
> > and straightfoward. Create an IOASID. Connect to a device. Fill the
> > IOASID with pages somehow. Worry about PASID labeling.
> > 
> > It really is critical to get all the vendor IOMMU people to go over it
> > and see how their HW features map into this.
> > 
> 
> Agree. btw I feel it might be good to have several design opens 
> centrally discussed after going through all the comments. Otherwise 
> they may be buried in different sub-threads and potentially with 
> insufficient care (especially for people who haven't completed the
> reading).
> 
> I summarized five opens here, about:
> 
> 1)  Finalizing the name to replace /dev/ioasid;
> 2)  Whether one device is allowed to bind to multiple IOASID fd's;
> 3)  Carry device information in invalidation/fault reporting uAPI;
> 4)  What should/could be specified when allocating an IOASID;
> 5)  The protocol between vfio group and kvm;
> 
> For 1), two alternative names are mentioned: /dev/iommu and 
> /dev/ioas. I don't have a strong preference and would like to hear 
> votes from all stakeholders. /dev/iommu is slightly better imho for 
> two reasons. First, per AMD's presentation in last KVM forum they 
> implement vIOMMU in hardware thus need to support user-managed 
> domains. An iommu uAPI notation might make more sense moving 
> forward. Second, it makes later uAPI naming easier as 'IOASID' can 
> be always put as an object, e.g. IOMMU_ALLOC_IOASID instead of 
> IOASID_ALLOC_IOASID. :)

I think two years ago I suggested /dev/iommu and it didn't go very far
at the time. We've also talked about this as /dev/sva for a while and
now /dev/ioasid

I think /dev/iommu is fine, and call the things inside them IOAS
objects.

Then we don't have naming aliasing with kernel constructs.
 
> For 2), Jason prefers to not blocking it if no kernel design reason. If 
> one device is allowed to bind multiple IOASID fd's, the main problem
> is about cross-fd IOASID nesting, e.g. having gpa_ioasid created in fd1 
> and giova_ioasid created in fd2 and then nesting them together (and

Huh? This can't happen

Creating an IOASID is an operation on on the /dev/ioasid FD. We won't
provide APIs to create a tree of IOASID's outside a single FD container.

If a device can consume multiple IOASID's it doesn't care how many or
what /dev/ioasid FDs they come from.

> To the other end there was also thought whether we should make
> a single I/O address space per IOASID fd. This was discussed in previous
> thread that #fd's are insufficient to afford theoretical 1M's address
> spaces per device. But let's have another revisit and draw a clear
> conclusion whether this option is viable.

I had remarks on this, I think per-fd doesn't work
 
> This implies that VFIO_BOUND_IOASID will be extended to allow user
> specify a device label. This label will be recorded in /dev/iommu to
> serve per-device invalidation request from and report per-device 
> fault data to the user.

I wonder which of the user providing a 64 bit cookie or the kernel
returning a small IDA is the best choice here? Both have merits
depending on what qemu needs..

> In addition, vPASID (if provided by user) will
> be also recorded in /dev/iommu so vPASID<->pPASID conversion 
> is conducted properly. e.g. invalidation request from user carries
> a vPASID which must be converted into pPASID before calling iommu
> driver. Vice versa for raw fault data which carries pPASID while the
> user expects a vPASID.

I don't think the PASID should be returned at all. It should return
the IOASID number in the FD and/or a u64 cookie associated with that
IOASID. Userspace should figure out what the IOASID & device
combination means.

> Seems to close this design open we have to touch the kAPI design. and 
> Joerg's input is highly appreciated here.

uAPI is forever, the kAPI is constantly changing. I always dislike
warping the uAPI based on the current kAPI situation.

Jason
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