RE: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

[Liu, Yi L]

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Tuesday, March 30, 2021 9:28 PM
> 
> On Tue, Mar 30, 2021 at 04:14:58AM +0000, Tian, Kevin wrote:
> 
> > One correction. The mdev should still construct the list of allowed PASID's
> as
> > you said (by listening to IOASID_BIND/UNBIND event), in addition to the
> ioasid
> > set maintained per VM (updated when a PASID is allocated/freed). The
> per-VM
> > set is required for inter-VM isolation (verified when a pgtable is bound to
> the
> > mdev/PASID), while the mdev's own list is necessary for intra-VM isolation
> when
> > multiple mdevs are assigned to the same VM (verified before loading a
> PASID
> > to the mdev). This series just handles the general part i.e. per-VM ioasid
> set and
> > leaves the mdev's own list to be managed by specific mdev driver which
> listens
> > to various IOASID events).
> 
> This is better, but I don't understand why we need such a convoluted
> design.
> 
> Get rid of the ioasid set.
>
> Each driver has its own list of allowed ioasids.

First, I agree with you it's necessary to have a per-device allowed ioasid
list. But besides it, I think we still need to ensure the ioasid used by a
VM is really allocated to this VM. A VM should not use an ioasid allocated
to another VM. right? Actually, this is the major intention for introducing
ioasid_set.

> Register a ioasid in the driver's list by passing the fd and ioasid #

The fd here is a device fd. Am I right? If yes, your idea is ioasid is
allocated via /dev/ioasid and associated with device fd via either VFIO
or vDPA ioctl. right? sorry I may be asking silly questions but really
need to ensure we are talking in the same page.

> No listening to events. A simple understandable security model.

For this suggestion, I have a little bit concern if we may have A-B/B-A
lock sequence issue since it requires the /dev/ioasid (if it supports)
to call back into VFIO/VDPA to check if the ioasid has been registered to
device FD and record it in the per-device list. right? Let's have more
discussion based on the skeleton sent by Kevin.

Regards,
Yi Liu