Re: Plan for /dev/ioasid RFC v2

[Jason Gunthorpe]

On Fri, Jun 25, 2021 at 10:27:18AM +0000, Tian, Kevin wrote:

> -   When receiving the binding call for the 1st device in a group, iommu_fd 
>     calls iommu_group_set_block_dma(group, dev->driver) which does 
>     several things:

The whole problem here is trying to match this new world where we want
devices to be in charge of their own IOMMU configuration and the old
world where groups are in charge.

Inserting the group fd and then calling a device-centric
VFIO_GROUP_GET_DEVICE_FD_NEW doesn't solve this conflict, and isn't
necessary. We can always get the group back from the device at any
point in the sequence do to a group wide operation.

What I saw as the appeal of the sort of idea was to just completely
leave all the difficult multi-device-group scenarios behind on the old
group centric API and then we don't have to deal with them at all, or
least not right away.

I'd see some progression where iommu_fd only works with 1:1 groups at
the start. Other scenarios continue with the old API.

Then maybe groups where all devices use the same IOASID.

Then 1:N groups if the source device is reliably identifiable, this
requires iommu subystem work to attach domains to sub-group objects -
not sure it is worthwhile.

But at least we can talk about each step with well thought out patches

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.

>         a) Check group viability. A group is viable only when all devices in
>             the group are in one of below states:
> 
>                 * driver-less
>                 * bound to a driver which is same as dev->driver (vfio in this case)
>                 * bound to an otherwise allowed driver (same list as in vfio)

This really shouldn't use hardwired driver checks. Attached drivers
should generically indicate to the iommu layer that they are safe for
iommu_fd usage by calling some function around probe()

Thus a group must contain only iommu_fd safe drivers, or drivers-less
devices before any of it can be used. It is the more general
refactoring of what VFIO is doing.

>         c) The iommu layer also verifies group viability on BUS_NOTIFY_
>             BOUND_DRIVER event. BUG_ON if viability is broken while block_dma
>             is set.

And with this concept of iommu_fd safety being first-class maybe we
can somehow eliminate this gross BUG_ON (and the 100's of lines of
code that are used to create it) by denying probe to non-iommu-safe
drivers, somehow.

> -   Binding other devices in the group to iommu_fd just succeeds since 
>     the group is already in block_dma.

I think the rest of this more or less describes the device centric
logic for multi-device groups we've already talked about. I don't
think it benifits from having the group fd

Jason