>-----Original Message-----
>From: Nicolin Chen <nicol...@nvidia.com>
>Subject: Re: [PATCH v5 05/21] vfio/iommufd: Force creating nested parent
>domain
>
>On Fri, Aug 22, 2025 at 02:40:43AM -0400, Zhenzhong Duan wrote:
>> Call pci_device_get_viommu_cap() to get if vIOMMU supports
>VIOMMU_CAP_HW_NESTED,
>> if yes, create nested parent domain which could be reused by vIOMMU to
>create
>> nested domain.
>>
>> Introduce helper vfio_device_viommu_get_nested to facilitate this
>> implementation.
>
>It'd be nicer to slightly mention the benefit of having it. Assuming
>that QEMU commit message can be as long as 80 characters:
>
>-------------------------
>Call pci_device_get_viommu_cap() to get if vIOMMU supports
>VIOMMU_CAP_HW_NESTED.
>
>If yes, create a nesting parent domain and add it to the container's hwpt_list,
>letting this parent domain cover the entire stage-2 mappings (gPA=>PA).
>
>This allows a VFIO passthrough device to directly attach to this default
>domain
>and then to use the system address space and its listener.
>
>Introduce a vfio_device_viommu_get_nested() helper to facilitate this
>implementation.
>-------------------------
Thanks, will do.
>
>> It is safe because even if VIOMMU_CAP_HW_NESTED is returned, s->flts is
>> forbidden and VFIO device fails in set_iommu_device() call, until we support
>> passthrough device with x-flts=on.
>
>I think this is too vendor specific to be mentioned here. Likely
>the previous VTD patch is the place to have this.
>
>Or you could say:
>
>--------------------------
>It is safe to do so because a vIOMMU will be able to fail in
>set_iommu_device()
>call, if something else related to the VFIO device or vIOMMU isn't compatible.
>--------------------------
Will do.
>
>> +bool vfio_device_viommu_get_nested(VFIODevice *vbasedev)
>> +{
>> + VFIOPCIDevice *vdev = vfio_pci_from_vfio_device(vbasedev);
>> +
>> + if (vdev) {
>> + return !!(pci_device_get_viommu_cap(&vdev->pdev) &
>> + VIOMMU_CAP_HW_NESTED);
>
>"get_nested" feels too general. Here it particularly means the cap:
>
>bool vfio_device_get_viommu_cap_hw_nested(VFIODevice *vbasedev)
Will use vfio_device_get_viommu_cap_hw_nested()
>
>> @@ -379,6 +379,14 @@ static bool
>iommufd_cdev_autodomains_get(VFIODevice *vbasedev,
>> flags = IOMMU_HWPT_ALLOC_DIRTY_TRACKING;
>> }
>>
>> + /*
>> + * If vIOMMU supports stage-1 translation, force to create nested
>parent
>
>"nested parent" is a contradictory phrase. Parent is a container
>holding some nested items. A nested parent sounds like a "parent"
>item that lives inside another parent container.
>
>In kernel kdoc/uAPI, we use:
> - "nesting parent" for stage-2 object
> - "nested hwpt", "nested domain" for stage-1 object
Thanks for sharing this info, I didn't notice that. I will fix the whole series
to use 'nesting parent'.
BRs,
Zhenzhong
