>-----Original Message-----
>From: Nicolin Chen <nicol...@nvidia.com>
>Subject: Re: [PATCH v3 02/20] hw/pci: Introduce
>pci_device_get_viommu_cap()
>
>On Wed, Jul 09, 2025 at 03:38:49AM +0000, Duan, Zhenzhong wrote:
>> >> +enum {
>> >> + VIOMMU_CAP_STAGE1 = BIT_ULL(0), /* stage1 page table
>> >supported */
>> >> +};
>> >
>> >Thanks for this work. I am happy to see that we can share the
>> >common code that allocates a NESTING_PARENT in the core using
>> >this flag.
>> >
>> >Yet on ARM, a STAGE1 page table isn't always a nested S1, the
>> >hardware accelerated one. More often, it can be just a regular
>> >1-stage translation table via emulated translation code and an
>> >emulated iotlb.
>>
>> What to return for an emulated device, VIOMMU_CAP_NESTED_S1 or 0?
>
>It would be ideally 0, since the get_viommu_cap is a per VFIO/PCI
>device op. But I see the caller of pci_device_get_viommu_cap() in
>this series has already done the identification between "emulated"
>and "passthrough" devices?
We do allow emulated PCI device calling pci_device_get_viommu_cap().
It's just no such request for now.
>
>That being said, it doesn't hurt to run it again in the callback
>function.
>
>> >I think this flag should indicate that the vIOMMU supports a
>> >HW-accelerated nested S1 HWPT allocation/invalidation.
>>
>> I'm OK to use VIOMMU_CAP_NESTED_S1 name. But still unclear which
>> checks should we have in .get_viommu_cap() callback to determine
>> returning VIOMMU_CAP_NESTED_S1 or not,
>> as we know nesting support is determined by host IOMMU not vIOMMU.
>
>I would say it's determined by both the host and vIOMMU.
>
>This is a common API that may return other caps too: eventually
>there can be a case where vIOMMU has its get_viommu_cap callback
>function but doesn't implement HW nesting via iommufd, even if
>the host IOMMU driver supports nesting.
OK, will use VIOMMU_CAP_NESTED_S1.
But for VTD, we check nesting compatibility in .set_iommu_device callback,
in . get_viommu_cap, we will return VIOMMU_CAP_NESTED_S1 if vIOMMU support it,
not checking for host IOMMU for simpler code.
Thanks
Zhenzhong
