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

[Jason Gunthorpe]

On Tue, Mar 30, 2021 at 01:37:05AM +0000, Tian, Kevin wrote:
> > 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.

This sounds like the exact opposite of what I'd like to see.

I do not want to see every subsystem gaining APIs to program a
PASID. All of that should be consolidated in *one place*.

I do not want to see VDPA and VFIO have two nearly identical sets of
APIs to control the PASID.

Drivers consuming a PASID, like VDPA, should consume the PASID and do
nothing more than authorize the HW to use it.

quemu should have general code under the viommu driver that drives
/dev/ioasid to create PASID's and manage the IO mapping according to
the guest's needs.

Drivers like VDPA and VFIO should simply accept that PASID and
configure/authorize their HW to do DMA's with its tag.

> 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:
> 
> - 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);
> - 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);
> - 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?

My reaction is to squash 'sva' and ioasid fds together, I can't see
why you'd need two fds to manipulate a PASID.

DEVICE_SET_SVA seems like the wrong language too, it should be more
like DEVICE_ALLOW_IOASID which only tells the iommu and driver to alow
the pci_device to use the IOASID.

Jason