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

Jason Gunthorpe
2021-04-07T12:20:42Z

On Wed, Apr 07, 2021 at 02:08:33AM +0000, Tian, Kevin wrote:

> > Because if you don't then we enter insane world where a PASID is being
> > created under /dev/ioasid but its translation path flows through setup
> > done by VFIO and the whole user API becomes an incomprehensible mess.
> >
> > How will you even associate the PASID with the other translation??
>
> PASID is attached to a specific iommu domain (created by VFIO/VDPA), which
> has GPA->HPA mappings already configured. If we view that mapping as an
> attribute of the iommu domain, it's reasonable to have the userspace-bound
> pgtable through /dev/ioasid to nest on it.

A user controlled page table should absolutely not be an attribute of
a hidden kernel object, nor should two parts of the kernel silently
connect to each other via a hidden internal objects like this.

Security is important - the kind of connection must use some explicit
FD authorization to access shared objects, not be made implicit!

IMHO this direction is a dead end for this reason.

> > The entire translation path for any ioasid or PASID should be defined
> > only by /dev/ioasid. Everything else is a legacy API.
> >
> > > If following your suggestion then VFIO must deny VFIO MAP operations
> > > on sva1 (assume userspace should not mix sva1 and sva2 in the same
> > > container and instead use /dev/ioasid to map for sva1)?
> >
> > No, userspace creates an iosaid for the guest physical mapping and
> > passes this ioasid to VFIO PCI which will assign it as the first layer
> > mapping on the RID
>
> Is it an dummy ioasid just for providing GPA mappings for nesting purpose
> of other IOASIDs? Then we waste one per VM?

Generic ioasid's are "free" they are just software constructs in the
kernel.

> > When PASIDs are allocated the uAPI will be told to logically nested
> > under the first ioasid. When VFIO authorizes a PASID for a RID it
> > checks that all the HW rules are being followed.
>
> As I explained above, why cannot we just use iommu domain to connect
> the dots?

Security.

> Every passthrough framework needs to create an iommu domain
> first. and It needs to support both devices w/ PASID and devices w/o
> PASID. For devices w/o PASID it needs to invent its own MAP
> interface anyway.

No, it should consume a ioasid from /dev/ioasid, use a common ioasid
map interface and assign that ioasid to a RID.

Don't get so fixated on PASID as a special case

> Then why do we bother creating another MAP interface through
> /dev/ioasid which not only duplicates but also creating transition
> burden between two set of MAP interfaces when the guest turns on/off
> the pasid capability on the device?

Don't transition. Always use the new interface. qemu detects the
kernel supports /dev/ioasid and *all iommu page table configuration*
goes through there. VFIO and VDPA APIs become unused for iommu
configuration.

> 'universally' upon from which angle you look at this problem. From IOASID
> p.o.v possibly yes, but from device passthrough p.o.v. it's the opposite
> since the passthrough framework needs to handle devices w/o PASID anyway
> (or even for device w/ PASID it could send traffic w/o PASID) thus 'universally'
> makes more sense if the passthrough framework can use one interface of its
> own to manage GPA mappings for all consumers (apply to the case when a
> PASID is allowed/authorized).

You correctly named it /dev/ioasid, it is a generic way to allocate,
manage and assign IOMMU page tables, which when generalized, only some
of which may consume a limited PASID.

RID and RID,PASID are the same thing, just a small difference in how
they match TLPs.

Jason