Re: [RFC] /dev/ioasid uAPI proposal

[Lu Baolu]

On 2021/6/1 15:15, Shenming Lu wrote:
> On 2021/6/1 13:10, Lu Baolu wrote:
> > Hi Shenming,
> >
> > On 6/1/21 12:31 PM, Shenming Lu wrote:
> > > On 2021/5/27 15:58, Tian, Kevin wrote:
> > > > /dev/ioasid provides an unified interface for managing I/O page tables for
> > > > devices assigned to userspace. Device passthrough frameworks (VFIO, vDPA,
> > > > etc.) are expected to use this interface instead of creating their own logic to
> > > > isolate untrusted device DMAs initiated by userspace.
> > > >
> > > > This proposal describes the uAPI of /dev/ioasid and also sample sequences
> > > > with VFIO as example in typical usages. The driver-facing kernel API provided
> > > > by the iommu layer is still TBD, which can be discussed after consensus is
> > > > made on this uAPI.
> > > >
> > > > It's based on a lengthy discussion starting from here:
> > > >      https://lore.kernel.org/linux-iommu/20210330132830.GO2356281@xxxxxxxxxx/
> > > >
> > > > It ends up to be a long writing due to many things to be summarized and
> > > > non-trivial effort required to connect them into a complete proposal.
> > > > Hope it provides a clean base to converge.
> > > [..]
> > >
> > > > /*
> > > >     * Page fault report and response
> > > >     *
> > > >     * This is TBD. Can be added after other parts are cleared up. Likely it
> > > >     * will be a ring buffer shared between user/kernel, an eventfd to notify
> > > >     * the user and an ioctl to complete the fault.
> > > >     *
> > > >     * The fault data is per I/O address space, i.e.: IOASID + faulting_addr
> > > >     */
> > > Hi,
> > >
> > > It seems that the ioasid has different usage in different situation, it could
> > > be directly used in the physical routing, or just a virtual handle that indicates
> > > a page table or a vPASID table (such as the GPA address space, in the simple
> > > passthrough case, the DMA input to IOMMU will just contain a Stream ID, no
> > > Substream ID), right?
> > >
> > > And Baolu suggested that since one device might consume multiple page tables,
> > > it's more reasonable to have one fault handler per page table. By this, do we
> > > have to maintain such an ioasid info list in the IOMMU layer?
> > As discussed earlier, the I/O page fault and cache invalidation paths
> > will have "device labels" so that the information could be easily
> > translated and routed.
> >
> > So it's likely the per-device fault handler registering API in iommu
> > core can be kept, but /dev/ioasid will be grown with a layer to
> > translate and propagate I/O page fault information to the right
> > consumers.
> Yeah, having a general preprocessing of the faults in IOASID seems to be
> a doable direction. But since there may be more than one consumer at the
> same time, who is responsible for registering the per-device fault handler?

The drivers register per page table fault handlers to /dev/ioasid which
will then register itself to iommu core to listen and route the per-
device I/O page faults. This is just a top level thought. Haven't gone
through the details yet. Need to wait and see what /dev/ioasid finally
looks like.

Best regards,
baolu