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

[Baolu Lu]

On 2023/5/31 2:50, Nicolin Chen wrote:
> Hi Baolu,
>
> On Tue, May 30, 2023 at 01:37:07PM +0800, Lu Baolu wrote:
>   
> > 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.
>
> I think that, whether the guest has an IOPF capability or not,
> the host should always forward any stage-1 fault/error back to
> the guest. Yet, the implementation of this series builds with
> the IOPF framework that doesn't report IOMMU_FAULT_DMA_UNRECOV.
>
> And I have my doubt at the using the IOPF framework with that
> IOMMU_PAGE_RESP_ASYNC flag: using the IOPF framework is for
> its bottom half workqueue, because a page response could take
> a long cycle. But adding that flag feels like we don't really
> need the bottom half workqueue, i.e. losing the point of using
> the IOPF framework, IMHO.
>
> Combining the two facts above, I wonder if we really need to
> go through the IOPF framework; can't we just register a user
> fault handler in the iommufd directly upon a valid event_fd?

Agreed. We should avoid workqueue in sva iopf framework. Perhaps we
could go ahead with below code? It will be registered to device with
iommu_register_device_fault_handler() in IOMMU_DEV_FEAT_IOPF enabling
path. Un-registering in the disable path of cause.

static int io_pgfault_handler(struct iommu_fault *fault, void *cookie)
{
        ioasid_t pasid = fault->prm.pasid;
        struct device *dev = cookie;
        struct iommu_domain *domain;

        if (fault->type != IOMMU_FAULT_PAGE_REQ)
                return -EOPNOTSUPP;

        if (fault->prm.flags & IOMMU_FAULT_PAGE_REQUEST_PASID_VALID)
                domain = iommu_get_domain_for_dev_pasid(dev, pasid, 0);
        else
                domain = iommu_get_domain_for_dev(dev);

        if (!domain || !domain->iopf_handler)
                return -ENODEV;

        if (domain->type == IOMMU_DOMAIN_SVA)
                return iommu_queue_iopf(fault, cookie);

        return domain->iopf_handler(fault, dev, domain->fault_data);
}

Best regards,
baolu