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

[Tian, Kevin]

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Tuesday, March 30, 2021 12:32 AM
> 
> On Wed, Mar 24, 2021 at 12:05:28PM -0700, Jacob Pan wrote:
> 
> > > IMHO a use created PASID is either bound to a mm (current) at creation
> > > time, or it will never be bound to a mm and its page table is under
> > > user control via /dev/ioasid.
> > >
> > True for PASID used in native SVA bind. But for binding with a guest mm,
> > PASID is allocated first (VT-d virtual cmd interface Spec 10.4.44), the
> > bind with the host IOMMU when vIOMMU PASID cache is invalidated.
> >
> > Our intention is to have two separate interfaces:
> > 1. /dev/ioasid (allocation/free only)
> > 2. /dev/sva (handles all SVA related activities including page tables)
> 
> I'm not sure I understand why you'd want to have two things. Doesn't
> that just complicate everything?
> 
> Manipulating the ioasid, including filling it with page tables, seems
> an integral inseperable part of the whole interface. Why have two ?

Hi, Jason,

Actually above is a major open while we are refactoring vSVA uAPI toward
this direction. We have two concerns about merging /dev/ioasid with
/dev/sva, and would like to hear your thought whether they are valid.

First, userspace may use ioasid in a non-SVA scenario where ioasid is 
bound to specific security context (e.g. a control vq in vDPA) instead of 
tying to mm. In this case there is no pgtable binding initiated from user
space. Instead, ioasid is allocated from /dev/ioasid and then programmed
to the intended security context through specific passthrough framework
which manages that context.

Second, ioasid is managed per process/VM while pgtable binding is a
device-wise operation.  The userspace flow looks like below for an integral
/dev/ioasid interface:

-----------initialization----------
- ioctl(container->fd, VFIO_SET_IOMMU, VFIO_TYPE1_NESTING_IOMMU)
- ioasid_fd = open(/dev/ioasid)
- ioctl(ioasid_fd, IOASID_GET_USVA_FD, &sva_fd) //an empty context
- ioctl(device->fd, VFIO_DEVICE_SET_SVA, &sva_fd); //sva_fd ties to device
- ioctl(sva_fd, USVA_GET_INFO, &sva_info);
-----------runtime----------------
- ioctl(ioasid_fd, IOMMU_ALLOC_IOASID, &ioasid);
- ioctl(sva_fd, USVA_BIND_PGTBL, &bind_data);
- ioctl(sva_fd, USVA_FLUSH_CACHE, &inv_info);
- ioctl(sva_fd, USVA_UNBIND_PGTBL, &unbind_data);
-----------destroy----------------
- ioctl(device->fd, VFIO_DEVICE_UNSET_SVA, &sva_fd);
- close(sva_fd)
- close(ioasid_fd)

Our hesitation here is based on one of your earlier comments that
you are not a fan of constructing fd's through ioctl. Are you OK with
above flow or have a better idea of handling it?

With separate interfaces then userspace just opens /dev/sva instead 
of getting it through ioasid_fd:

- ioasid_fd = open(/dev/ioasid)
- sva_fd = open(/dev/sva)

Thanks
Kevin