Documentation: userspace-api: iommufd: Update vIOMMU

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

Link: https://patch.msgid.link/r/7e4302064e0d02137c1b1e139342affc0485ed3f.1730836219.git.nicolinc@nvidia.com
Reviewed-by: Jason Gunthorpe <[email protected]>
Reviewed-by: Kevin Tian <[email protected]>
Signed-off-by: Nicolin Chen <[email protected]>
Reviewed-by: Bagas Sanjaya <[email protected]>
Signed-off-by: Jason Gunthorpe <[email protected]>

Author: nicolinc

Documentation/userspace-api/iommufd.rst

@@ -63,6 +63,39 @@ Following IOMMUFD objects are exposed to userspace:
   space usually has mappings from guest-level I/O virtual addresses to guest-
   level physical addresses.
 
+- IOMMUFD_OBJ_VIOMMU, representing a slice of the physical IOMMU instance,
+  passed to or shared with a VM. It may be some HW-accelerated virtualization
+  features and some SW resources used by the VM. For examples:
+
+  * Security namespace for guest owned ID, e.g. guest-controlled cache tags
+  * Non-device-affiliated event reporting, e.g. invalidation queue errors
+  * Access to a sharable nesting parent pagetable across physical IOMMUs
+  * Virtualization of various platforms IDs, e.g. RIDs and others
+  * Delivery of paravirtualized invalidation
+  * Direct assigned invalidation queues
+  * Direct assigned interrupts
+
+  Such a vIOMMU object generally has the access to a nesting parent pagetable
+  to support some HW-accelerated virtualization features. So, a vIOMMU object
+  must be created given a nesting parent HWPT_PAGING object, and then it would
+  encapsulate that HWPT_PAGING object. Therefore, a vIOMMU object can be used
+  to allocate an HWPT_NESTED object in place of the encapsulated HWPT_PAGING.
+
+  .. note::
+
+     The name "vIOMMU" isn't necessarily identical to a virtualized IOMMU in a
+     VM. A VM can have one giant virtualized IOMMU running on a machine having
+     multiple physical IOMMUs, in which case the VMM will dispatch the requests
+     or configurations from this single virtualized IOMMU instance to multiple
+     vIOMMU objects created for individual slices of different physical IOMMUs.
+     In other words, a vIOMMU object is always a representation of one physical
+     IOMMU, not necessarily of a virtualized IOMMU. For VMMs that want the full
+     virtualization features from physical IOMMUs, it is suggested to build the
+     same number of virtualized IOMMUs as the number of physical IOMMUs, so the
+     passed-through devices would be connected to their own virtualized IOMMUs
+     backed by corresponding vIOMMU objects, in which case a guest OS would do
+     the "dispatch" naturally instead of VMM trappings.
+
 All user-visible objects are destroyed via the IOMMU_DESTROY uAPI.
 
 The diagrams below show relationships between user-visible objects and kernel
@@ -101,6 +134,28 @@ creating the objects and links::
            |------------>|iommu_domain|<----|iommu_domain|<----|device|
                          |____________|     |____________|     |______|
 
+  _______________________________________________________________________
+ |                      iommufd (with vIOMMU)                            |
+ |                                                                       |
+ |                             [5]                                       |
+ |                        _____________                                  |
+ |                       |             |                                 |
+ |      |----------------|    vIOMMU   |                                 |
+ |      |                |             |                                 |
+ |      |                |             |                                 |
+ |      |      [1]       |             |          [4]             [2]    |
+ |      |     ______     |             |     _____________     ________  |
+ |      |    |      |    |     [3]     |    |             |   |        | |
+ |      |    | IOAS |<---|(HWPT_PAGING)|<---| HWPT_NESTED |<--| DEVICE | |
+ |      |    |______|    |_____________|    |_____________|   |________| |
+ |      |        |              |                  |               |     |
+ |______|________|______________|__________________|_______________|_____|
+        |        |              |                  |               |
+  ______v_____   |        ______v_____       ______v_____       ___v__
+ |   struct   |  |  PFN  |  (paging)  |     |  (nested)  |     |struct|
+ |iommu_device|  |------>|iommu_domain|<----|iommu_domain|<----|device|
+ |____________|   storage|____________|     |____________|     |______|
+
 1. IOMMUFD_OBJ_IOAS is created via the IOMMU_IOAS_ALLOC uAPI. An iommufd can
    hold multiple IOAS objects. IOAS is the most generic object and does not
    expose interfaces that are specific to single IOMMU drivers. All operations
@@ -132,7 +187,8 @@ creating the objects and links::
      flag is set.
 
 4. IOMMUFD_OBJ_HWPT_NESTED can be only manually created via the IOMMU_HWPT_ALLOC
-   uAPI, provided an hwpt_id via @pt_id to associate the new HWPT_NESTED object
+   uAPI, provided an hwpt_id or a viommu_id of a vIOMMU object encapsulating a
+   nesting parent HWPT_PAGING via @pt_id to associate the new HWPT_NESTED object
    to the corresponding HWPT_PAGING object. The associating HWPT_PAGING object
    must be a nesting parent manually allocated via the same uAPI previously with
    an IOMMU_HWPT_ALLOC_NEST_PARENT flag, otherwise the allocation will fail. The
@@ -149,6 +205,18 @@ creating the objects and links::
       created via the same IOMMU_HWPT_ALLOC uAPI. The difference is at the type
       of the object passed in via the @pt_id field of struct iommufd_hwpt_alloc.
 
+5. IOMMUFD_OBJ_VIOMMU can be only manually created via the IOMMU_VIOMMU_ALLOC
+   uAPI, provided a dev_id (for the device's physical IOMMU to back the vIOMMU)
+   and an hwpt_id (to associate the vIOMMU to a nesting parent HWPT_PAGING). The
+   iommufd core will link the vIOMMU object to the struct iommu_device that the
+   struct device is behind. And an IOMMU driver can implement a viommu_alloc op
+   to allocate its own vIOMMU data structure embedding the core-level structure
+   iommufd_viommu and some driver-specific data. If necessary, the driver can
+   also configure its HW virtualization feature for that vIOMMU (and thus for
+   the VM). Successful completion of this operation sets up the linkages between
+   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.
+
 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).
 
@@ -161,6 +229,7 @@ User visible objects are backed by following datastructures:
 - iommufd_device for IOMMUFD_OBJ_DEVICE.
 - iommufd_hwpt_paging for IOMMUFD_OBJ_HWPT_PAGING.
 - iommufd_hwpt_nested for IOMMUFD_OBJ_HWPT_NESTED.
+- iommufd_viommu for IOMMUFD_OBJ_VIOMMU.
 
 Several terminologies when looking at these datastructures: