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    SubjectRe: [RFC PATCHES 00/17] IOMMUFD: Deliver IO page faults to user space
    On Sun, Jun 25, 2023 at 02:30:46PM +0800, Baolu Lu wrote:
    > External email: Use caution opening links or attachments
    >
    >
    > 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.

    Well, for a virtualization use case, I still think it's should
    be registered in iommufd. Having a device without an IOPF/PRI
    capability, a guest OS should receive some faults too, if that
    device causes a translation failure.

    And for a vSVA use case, the IOMMU_DEV_FEAT_IOPF feature only
    gets enabled in the guest VM right? How could the host enable
    the IOMMU_DEV_FEAT_IOPF to trigger this handler?

    Thanks
    Nic

    > 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

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