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

[Jason Gunthorpe]

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.

> 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.

Jason