Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

Jason Gunthorpe
2021-04-01T16:03:37Z

On Thu, Apr 01, 2021 at 02:08:17PM +0000, Liu, Yi L wrote:

> DMA page faults are delivered to root-complex via page request message and
> it is per-device according to PCIe spec. Page request handling flow is:
>
> 1) iommu driver receives a page request from device
> 2) iommu driver parses the page request message. Get the RID,PASID, faulted
> page and requested permissions etc.
> 3) iommu driver triggers fault handler registered by device driver with
> iommu_report_device_fault()

This seems confused.

The PASID should define how to handle the page fault, not the driver.

I don't remember any device specific actions in ATS, so what is the
driver supposed to do?

> 4) device driver's fault handler signals an event FD to notify userspace to
> fetch the information about the page fault. If it's VM case, inject the
> page fault to VM and let guest to solve it.

If the PASID is set to 'report page fault to userspace' then some
event should come out of /dev/ioasid, or be reported to a linked
eventfd, or whatever.

If the PASID is set to 'SVM' then the fault should be passed to
handle_mm_fault

And so on.

Userspace chooses what happens based on how they configure the PASID
through /dev/ioasid.

Why would a device driver get involved here?

> Eric has sent below series for the page fault reporting for VM with passthru
> device.
> https://lore.kernel.org/kvm/20210223210625.604517-5-eric.auger-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org/

It certainly should not be in vfio pci. Everything using a PASID needs
this infrastructure, VDPA, mdev, PCI, CXL, etc.

Jason