RE: [PATCH v2 2/4] iommu: Add new iommu op to get iommu hardware information

["Liu, Yi L" <yi.l.liu@intel.com>]

> From: Tian, Kevin <kevin.tian@intel.com>
> Sent: Thursday, March 16, 2023 4:17 PM
> 
> > From: Liu, Yi L <yi.l.liu@intel.com>
> > Sent: Thursday, March 9, 2023 3:54 PM
> > @@ -222,6 +223,11 @@ struct iommu_iotlb_gather {
> >  /**
> >   * struct iommu_ops - iommu ops and capabilities
> >   * @capable: check capability
> > + * @hw_info: IOMMU hardware information. The type of the returned
> data
> > is
> > + *           defined in include/uapi/linux/iommufd.h. The data buffer is
> 
> "The type of the returned data is marked by @driver_type".
> 
> "defined in include/uapi/linux/iommufd.h" should belong to the comment
> of @driver_type

Sure.

> 
> > + *           allocated in the IOMMU driver and the caller should free it
> > + *           after use. Return the data buffer if success, or ERR_PTR on
> > + *           failure.
> >   * @domain_alloc: allocate iommu domain
> >   * @probe_device: Add device to iommu driver handling
> >   * @release_device: Remove device from iommu driver handling
> > @@ -246,11 +252,17 @@ struct iommu_iotlb_gather {
> >   * @remove_dev_pasid: Remove any translation configurations of a
> specific
> >   *                    pasid, so that any DMA transactions with this pasid
> >   *                    will be blocked by the hardware.
> > + * @driver_type: One of enum iommu_hw_info_type. This is used in the
> > hw_info
> > + *               reporting path. For the drivers that supports it, a unique
> > + *               type should be defined. For the driver that does not support
> > + *               it, this field is the IOMMU_HW_INFO_TYPE_DEFAULT that is 0.
> > + *               Hence, such drivers do not need to care this field.
> 
> The meaning of "driver_type" is much broader than reporting hw_info.
> 
> let's be accurate to call it as "hw_info_type". and while we have two
> separate fields for one feature where is the check enforced on whether
> both are provided?

It is filled in the uapi structure by referring ops->driver_type in next
patch.

> Is it simpler to return the type directly in @hw_info?

Per the current description, if the iommu driver doesn't implement .hw_info
callback, then it will not set driver_type field neither. Then this field is 0
(IOMMU_HW_INFO_TYPE_NONE). The GET_HW_INFO ioctl in next patch
would fail as well. Under this implementation, returning the driver_type
(a.k.a hw_info_type per your comment) in the hw_info callback may be
simpler.

But I plan to update the implementation per the below remark from Jason.
The GET_HW_INFO needs to succeed even if the underlying iommu driver
does not implement hw_info callback. If so, it's still much more convenient
to get the type by referring ops->driver_type.

https://lore.kernel.org/kvm/ZAcwJSK%2F9UVI9LXu@nvidia.com/

Also, per Nic's other remark, there would be a bitmap named hwpt_types
field added to iommu_ops. Then it is also easier to referring it by
ops->hwpt_types.

https://lore.kernel.org/linux-iommu/ZArgAXMUpNjDfFgZ@Asurada-Nvidia/#t

Surely, we also have another alternative. We can enforce all the iommu
drivers to implement a minimum hw_info callback which just returns the
driver_type if it does not have driver-specific data to report to the user
yet.

> btw IOMMU_HW_INFO_TYPE_DEFAULT also sounds misleading.
> 'default' implies hw_info still available but in a default format.
> 
> probably it's clearer to call it IOMMU_HW_INFO_TYPE_NONE.

Sure. Makes sense. So _NONE means no driver specific info is
Reported back to user.

Regards,
Yi Liu