Re: Plan for /dev/ioasid RFC v2

[Jason Gunthorpe]

On Mon, Jun 14, 2021 at 03:09:31AM +0000, Tian, Kevin wrote:

> If a device can be always blocked from accessing memory in the IOMMU
> before it's bound to a driver or more specifically before the driver
> moves it to a new security context, then there is no need for VFIO
> to track whether IOASIDfd has taken over ownership of the DMA
> context for all devices within a group.

I've been assuming we'd do something like this, where when a device is
first turned into a VFIO it tells the IOMMU layer that this device
should be DMA blocked unless an IOASID is attached to
it. Disconnecting an IOASID returns it to blocked.

> If this works I didn't see the need for vfio to keep the sequence. 
> VFIO still keeps group fd to claim ownership of all devices in a 
> group.

As Alex says you still have to deal with the problem that device A in
a group can gain control of device B in the same group.

This means device A and B can not be used from to two different
security contexts.

If the /dev/iommu FD is the security context then the tracking is
needed there.

Jason