Documentation: userspace-api: iommufd: Update vDEVICE

With the introduction of the new object and its infrastructure, update the
doc and the vIOMMU graph to reflect that.

Link: https://patch.msgid.link/r/e1ff278b7163909b2641ae04ff364bb41d2a2a2e.1730836308.git.nicolinc@nvidia.com
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

Author: nicolinc

Documentation/userspace-api/iommufd.rst

@@ -96,6 +96,19 @@ Following IOMMUFD objects are exposed to userspace:
      backed by corresponding vIOMMU objects, in which case a guest OS would do
      the "dispatch" naturally instead of VMM trappings.
 
+- IOMMUFD_OBJ_VDEVICE, representing a virtual device for an IOMMUFD_OBJ_DEVICE
+  against an IOMMUFD_OBJ_VIOMMU. This virtual device holds the device's virtual
+  information or attributes (related to the vIOMMU) in a VM. An immediate vDATA
+  example can be the virtual ID of the device on a vIOMMU, which is a unique ID
+  that VMM assigns to the device for a translation channel/port of the vIOMMU,
+  e.g. vSID of ARM SMMUv3, vDeviceID of AMD IOMMU, and vRID of Intel VT-d to a
+  Context Table. Potential use cases of some advanced security information can
+  be forwarded via this object too, such as security level or realm information
+  in a Confidential Compute Architecture. A VMM should create a vDEVICE object
+  to forward all the device information in a VM, when it connects a device to a
+  vIOMMU, which is a separate ioctl call from attaching the same device to an
+  HWPT_PAGING that the vIOMMU holds.
+
 All user-visible objects are destroyed via the IOMMU_DESTROY uAPI.
 
 The diagrams below show relationships between user-visible objects and kernel
@@ -135,16 +148,16 @@ creating the objects and links::
                          |____________|     |____________|     |______|
 
   _______________________________________________________________________
- |                      iommufd (with vIOMMU)                            |
+ |                      iommufd (with vIOMMU/vDEVICE)                    |
  |                                                                       |
- |                             [5]                                       |
- |                        _____________                                  |
- |                       |             |                                 |
- |      |----------------|    vIOMMU   |                                 |
- |      |                |             |                                 |
- |      |                |             |                                 |
- |      |      [1]       |             |          [4]             [2]    |
- |      |     ______     |             |     _____________     ________  |
+ |                             [5]                [6]                    |
+ |                        _____________      _____________               |
+ |                       |             |    |             |              |
+ |      |----------------|    vIOMMU   |<---|   vDEVICE   |<----|        |
+ |      |                |             |    |_____________|     |        |
+ |      |                |             |                        |        |
+ |      |      [1]       |             |          [4]           | [2]    |
+ |      |     ______     |             |     _____________     _|______  |
  |      |    |      |    |     [3]     |    |             |   |        | |
  |      |    | IOAS |<---|(HWPT_PAGING)|<---| HWPT_NESTED |<--| DEVICE | |
  |      |    |______|    |_____________|    |_____________|   |________| |
@@ -217,6 +230,15 @@ creating the objects and links::
    the vIOMMU object and the HWPT_PAGING, then this vIOMMU object can be used
    as a nesting parent object to allocate an HWPT_NESTED object described above.
 
+6. IOMMUFD_OBJ_VDEVICE can be only manually created via the IOMMU_VDEVICE_ALLOC
+   uAPI, provided a viommu_id for an iommufd_viommu object and a dev_id for an
+   iommufd_device object. The vDEVICE object will be the binding between these
+   two parent objects. Another @virt_id will be also set via the uAPI providing
+   the iommufd core an index to store the vDEVICE object to a vDEVICE array per
+   vIOMMU. If necessary, the IOMMU driver may choose to implement a vdevce_alloc
+   op to init its HW for virtualization feature related to a vDEVICE. Successful
+   completion of this operation sets up the linkages between vIOMMU and device.
+
 A device can only bind to an iommufd due to DMA ownership claim and attach to at
 most one IOAS object (no support of PASID yet).
 
@@ -230,6 +252,7 @@ User visible objects are backed by following datastructures:
 - iommufd_hwpt_paging for IOMMUFD_OBJ_HWPT_PAGING.
 - iommufd_hwpt_nested for IOMMUFD_OBJ_HWPT_NESTED.
 - iommufd_viommu for IOMMUFD_OBJ_VIOMMU.
+- iommufd_vdevice for IOMMUFD_OBJ_VDEVICE.
 
 Several terminologies when looking at these datastructures: