RE: Plan for /dev/ioasid RFC v2

From: Tian, Kevin
Date: Sun Jun 27 2021 - 22:04:12 EST

Hi, Jason,

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Friday, June 25, 2021 10:36 PM
> The only thing that needs to be done to get the 1:1 step is to broadly
> define how the other two cases will work so we don't get into trouble
> and set some way to exclude the problematic cases from even getting to
> iommu_fd in the first place.
> For instance if we go ahead and create /dev/vfio/device nodes we could
> do this only if the group was 1:1, otherwise the group cdev has to be
> used, along with its API.

I may misunderstand your position in last reply.

The bottom line is that iommu fd uAPI and helper functions should be
device-centric (no group fd carried). This is what this sketch tries to

However I'm getting confused on your position on vfio uAPIs.

At some point I feel you are OK to keep vfio group fd:

"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."

But you are also obviously against faking group for mdev.

Combining with the last paragraph above, are you actually suggesting
that 1:1 group (including mdev) should use a new device-centric vfio
uAPI (without group fd) while existing group-centric vfio uAPI is only
kept for 1:N group (with slight semantics change in my sketch to match
device-centric iommu fd API)?

If yes, some assumptions in this sketch might be changed. For example,
with /dev/vfio/device node I'm not sure how the user can pass {iommu_fd,
device_cookie} to establish the security context when opening the node
(not via an indirect group ioctl). Then it implies that we may have to allow
the user open a device before it is put into a security context, thus the
safe guard may have to be enabled on mmap() for 1:1 group. This is a
different sequence from the existing group-centric model.

Anyway, appreciate if you can elaborate your thoughts so we can further
think about them.