Support multiple allocations for vGPUs

Removing the TODO that only allows one VGPU allocation per instance. Now we no
longer need to support the very old VGPU usage for the root provider, this
is easy.

Change-Id: I48d2b700049c81071710e37c05579239255c3539
Related-Bug: #1758086
Signed-off-by: Sylvain Bauza <sbauza@redhat.com>

doc/source/admin/virtual-gpu.rst

@@ -172,6 +172,39 @@ provided by compute nodes.
    $ openstack server create --flavor vgpu_1 --image cirros-0.3.5-x86_64-uec --wait test-vgpu
 
 
+Ask for more than one vGPU per instance by the flavor
+-----------------------------------------------------
+
+.. versionchanged:: 33.0.0
+
+We have an open bug report `bug 1758086`_ explaining that the nvidia driver
+doesn't support more than one vGPU per instance (and per GPU resource - which
+can be a physical GPU or a virtual function, see nvidia docs for more details).
+In order to alleviate this problem, this is mandatory to require in the flavor
+to have all the vGPUs to be spread between multiple GPU resource providers.
+
+For example, you can request two groups of vGPUs this way :
+
+.. code-block:: console
+
+   $ openstack flavor set vgpu_2 --property "resources1:VGPU=1" \
+                                 --property "resources2:VGPU=1" \
+
+
+With SR-IOV GPUs (you may need to refer to nvidia documentation to know the
+distinction), this will work without requiring further attributes as every
+single VGPU Resource Provider only provides a single VGPU resource.
+
+For non-SRIOV GPUs, you may require other properties in order to request
+Placement to allocate you some host with two distinct GPUs.
+You may need to create distinct custom traits per GPU or custom resource
+classes for explicitly telling in your flavor that you would want resources
+from distinct entities, or you could use ``group_policy=isolate`` as a property
+but you would need to make sure that you don't ask for other resources but
+virtual GPUs in your flavor or Placement would shard all the allocations for
+*all* resource groups.
+
+
 How to discover a GPU type
 --------------------------
 
@@ -490,6 +523,7 @@ For nested vGPUs:
 .. _bug 1762688: https://bugs.launchpad.net/nova/+bug/1762688
 .. _bug 1948705: https://bugs.launchpad.net/nova/+bug/1948705
 .. _supports vGPU live-migrations: https://specs.openstack.org/openstack/nova-specs/specs/2024.1/approved/libvirt-mdev-live-migrate.html
+.. _bug 1758086: https://bugs.launchpad.net/nova/+bug/1758086
 
 .. Links
 .. _Intel GVT-g: https://01.org/igvt-g