Re: [RFC PATCHES 00/17] IOMMUFD: Deliver IO page faults to user space

[Baolu Lu]

On 5/31/23 8:33 AM, Jason Gunthorpe wrote:
> On Tue, May 30, 2023 at 01:37:07PM +0800, Lu Baolu wrote:
> > Hi folks,
> >
> > This series implements the functionality of delivering IO page faults to
> > user space through the IOMMUFD framework. The use case is nested
> > translation, where modern IOMMU hardware supports two-stage translation
> > tables. The second-stage translation table is managed by the host VMM
> > while the first-stage translation table is owned by the user space.
> > Hence, any IO page fault that occurs on the first-stage page table
> > should be delivered to the user space and handled there. The user space
> > should respond the page fault handling result to the device top-down
> > through the IOMMUFD response uAPI.
> >
> > User space indicates its capablity of handling IO page faults by setting
> > a user HWPT allocation flag IOMMU_HWPT_ALLOC_FLAGS_IOPF_CAPABLE. IOMMUFD
> > will then setup its infrastructure for page fault delivery. Together
> > with the iopf-capable flag, user space should also provide an eventfd
> > where it will listen on any down-top page fault messages.
> >
> > On a successful return of the allocation of iopf-capable HWPT, a fault
> > fd will be returned. User space can open and read fault messages from it
> > once the eventfd is signaled.
>
> This is a performance path so we really need to think about this more,
> polling on an eventfd and then reading a different fd is not a good
> design.
>
> What I would like is to have a design from the start that fits into
> io_uring, so we can have pre-posted 'recvs' in io_uring that just get
> completed at high speed when PRIs come in.
>
> This suggests that the PRI should be delivered via read() on a single
> FD and pollability on the single FD without any eventfd.

Good suggestion. I will head in this direction.

> > Besides the overall design, I'd like to hear comments about below
> > designs:
> >
> > - The IOMMUFD fault message format. It is very similar to that in
> >    uapi/linux/iommu which has been discussed before and partially used by
> >    the IOMMU SVA implementation. I'd like to get more comments on the
> >    format when it comes to IOMMUFD.
>
> We have to have the same discussion as always, does a generic fault
> message format make any sense here?
>
> PRI seems more likely that it would but it needs a big carefull cross
> vendor check out.

Yeah, good point.

As far as I can see, there are at least three types of IOPF hardware
implementation.

- PCI/PRI: Vendors might have their own additions. For example, VT-d 3.0
  allows root-complex integrated endpoints to carry device specific
  private data in their page requests. This has been removed from the
  spec since v4.0.

- DMA stalls.

- Device-specific (non-PRI, not through IOMMU).

Does IOMMUFD want to support the last case?

Best regards,
baolu