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

Jason Gunthorpe
2021-04-01T00:37:05Z

On Wed, Mar 31, 2021 at 04:46:21PM -0700, Jacob Pan wrote:
> Hi Jason,
>
> On Wed, 31 Mar 2021 09:38:01 -0300, Jason Gunthorpe <jgg-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org> wrote:
>
> > > > Get rid of the ioasid set.
> > > >
> > > > Each driver has its own list of allowed ioasids.
> > [...]
> >
> > The /dev/ioasid FD replaces this security check. By becoming FD
> > centric you don't need additional kernel security objects.
> >
> > Any process with access to the /dev/ioasid FD is allowed to control
> > those PASID. The seperation between VMs falls naturally from the
> > seperation of FDs without creating additional, complicated, security
> > infrastrucure in the kernel.
> >
> > This is why all APIs must be FD focused, and you need to have a
> > logical layering of responsibility.
> >
> > Allocate a /dev/ioasid FD
> > Allocate PASIDs inside the FD
> > Assign memory to the PASIDS
> >
> > Open a device FD, eg from VFIO or VDP
> > Instruct the device FD to authorize the device to access PASID A in
> > an ioasid FD
> How do we know user provided PASID A was allocated by the ioasid FD?

You pass in the ioasid FD and use a 'get pasid from fdno' API to
extract the required kernel structure.

> Shouldn't we validate user input by tracking which PASIDs are
> allocated by which ioasid FD?

Yes, but it is integral to the ioasid FD, not something separated.

> > VFIO extracts some kernel representation of the ioasid from the ioasid
> > fd using an API
> >
> This lookup API seems to be asking for per ioasid FD storage array. Today,
> the ioasid_set is per mm and contains a Xarray.

Right, put the xarray per FD. A set per mm is fairly nonsensical, we
don't use the mm as that kind of security key.

> Since each VM, KVM can only open one ioasid FD, this per FD array
> would be equivalent to the per mm ioasid_set, right?

Why only one? Each interaction with the other FDs should include the
PASID/FD pair. There is no restriction to just one.

> > VFIO does some kernel call to IOMMU/IOASID layer that says 'tell the
> > IOMMU that this PCI device is allowed to use this PASID'
>
> Would it be redundant to what iommu_uapi_sva_bind_gpasid() does? I thought
> the idea is to use ioasid FD IOCTL to issue IOMMU uAPI calls. Or we can
> skip this step for now and wait for the user to do SVA bind.

I'm not sure what you are asking.

Possibly some of the IOMMU API will need a bit adjusting to make
things split.

The act of programming the page tables and the act of authorizing a
PCI BDF to use a PASID are distinct things with two different IOCTLs.

iommu_uapi_sva_bind_gpasid() is never called by anything, and it's
uAPI is never implemented.

Joerg? Why did you merge dead uapi and dead code?

Jason