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

Jason Gunthorpe
2021-04-15T23:07:32Z

On Thu, Apr 15, 2021 at 03:11:19PM +0200, Auger Eric wrote:
> Hi Jason,
>
> On 4/1/21 6:03 PM, Jason Gunthorpe wrote:
> > 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.
>
> In my series I don't use PASID at all. I am just enabling nested stage
> and the guest uses a single context. I don't allocate any user PASID at
> any point.
>
> When there is a fault at physical level (a stage 1 fault that concerns
> the guest), this latter needs to be reported and injected into the
> guest. The vfio pci driver registers a fault handler to the iommu layer
> and in that fault handler it fills a circ bugger and triggers an eventfd
> that is listened to by the VFIO-PCI QEMU device. this latter retrives
> the faault from the mmapped circ buffer, it knowns which vIOMMU it is
> attached to, and passes the fault to the vIOMMU.
> Then the vIOMMU triggers and IRQ in the guest.
>
> We are reusing the existing concepts from VFIO, region, IRQ to do that.
>
> For that use case, would you also use /dev/ioasid?

/dev/ioasid could do all the things you described vfio-pci as doing,
it can even do them the same way you just described.

Stated another way, do you plan to duplicate all of this code someday
for vfio-cxl? What about for vfio-platform? ARM SMMU can be hooked to
platform devices, right?

I feel what you guys are struggling with is some choice in the iommu
kernel APIs that cause the events to be delivered to the pci_device
owner, not the PASID owner.

That feels solvable.

Jason