Re: [PATCH rfcv3 15/21] intel_iommu: Bind/unbind guest page table to host

[Nicolin Chen <nicolinc@nvidia.com>]

On Mon, Jun 16, 2025 at 03:24:06AM +0000, Duan, Zhenzhong wrote:
> Hi @Liu, Yi L @Nicolin Chen, for emulated/passthru devices
> behind the same pcie-pci bridge, I think of an idea, adding
> a new PCI callback:
> 
> AddressSpace * (*get_address_space_extend)(PCIBus *bus, 
> void *opaque, int devfn, bool accel_dev);
>
> which pass in real bus/devfn and a new param accel_dev which
> is true for vfio device.

Just =y for all vfio (passthrough) devices?

ARM tentatively does this for get_address_space using Shameer's
trick to detect if the device is a passthrough VFIO one:

    PCIDevice *pdev = pci_find_device(bus, pci_bus_num(bus), devfn);
    bool has_iommufd = !!object_property_find(OBJECT(pdev), "iommufd");

    if (smmu->nested && ... && has_iommufd) {
        return &sdev->as_sysmem;
    }

So, I guess "accel_dev" could be just:
    !!object_property_find(OBJECT(pdev), "iommufd")
?

> Vtd implements this callback and return separate AS for vfio
> device if it's under an pcie-pci bridge and flts=on;
> otherwise it fallback to call .get_address_space(). This way
> emulated devices and passthru devices behind the same pcie-pci
> bridge can have different AS.

Again, if "vfio-device" tag with "iommufd" property is enough to
identify devices to separate their address spaces, perhaps the
existing get_address_space is enough.

> If above idea is acceptable, then only obstacle is ERRATA_772415,
> maybe we can let VFIO check this errata and bypass RO mapping from
> beginning?

Yes. There can be some communication between vIOMMU and the VFIO
core.

> Or we just block this VFIO device running with flts=on if
> ERRATA_772415 and suggesting running with flts=off?

That sounds like a simpler solution, so long as nobody complains
about this limitation :)

Thanks
Nicolin