libvirt: Supporting multiple vGPU types for a single pGPU

https://blueprints.launchpad.net/nova/+spec/vgpu-rocky

Virtual GPUs in Nova was implemented in Queens but only with one supported GPU type per compute node. Now that Nested Resource Providers is a thing, this spec is discussing about how to have one supported vGPU type per physical GPU (given a GPU card can have multiple physical GPUs).

Note

As Xen provides a specific feature where physical GPUs supporting same vGPU type are within a single pGPU group, that virt driver doesn’t need to know which exact pGPUs need to support a specific type, hence this spec only targets the libvirt driver.

Problem description

Hardware vendors tell us that a physical GPU device can support multiple types. That said, Intel (to be confirmed) and NVidia vendor drivers only accept one type for all the virtual devices per graphical processing unit.

For example, NVidia GRID physical cards can accept a list of different GPU types, but the driver can only support one type per physical GPU.

Consequently, we require a way to instruct the libvirt driver which vGPU types an NVIDIA or Intel physical GPU is configured to accept.

Use Cases

An operator needs a way to inform the libvirt driver which vGPU types an NVIDIA or Intel physical GPU is configured to accept.

Proposed change

We already have [devices]/enabled_vgpu_types that define which types the Nova compute node can use:

[devices]
enabled_vgpu_types = [str_vgpu_type_1, str_vgpu_type_2, ...]

Now we propose that libvirt will accept configuration sections that are related to the [devices]/enabled_vgpu_types and specifies which exact pGPUs are related to the enabled vGPU types and will have a device_addresses option defined like this:

cfg.ListOpt('device_addresses',
            default=[],
            help="""
List of physical PCI addresses to associate with a specific GPU type.

The particular physical GPU device address needs to be mapped to the vendor
vGPU type which that physical GPU is configured to accept. In order to
provide this mapping, there will be a CONF section with a name corresponding
to the following template: "vgpu_type_%(vgpu_type_name)s

The vGPU type to associate with the PCI devices has to be the section name
prefixed by ``vgpu_``. For example, for 'nvidia-11', you would declare
``[vgpu_nvidia-11]/device_addresses``.

Each vGPU type also has to be declared in ``[devices]/enabled_vgpu_types``.

Related options:

* ``[devices]/enabled_vgpu_types``
"""),

For example, it would be set in nova.conf:

[devices]
enabled_vgpu_types = nvidia-35,nvidia-36
[vgpu_nvidia-35]
device_addresses = 0000:84:00.0,0000:85:00.0
[vgpu_nvidia-36]
device_addresses = 0000:86:00.0

In that case, the nvidia-35 vGPU type would be supported by the physical GPUs that are in the PCI addresses 0000:84:00.0 and 0000:85:00.0, while nvidia-36 vGPU would only be supported by 0000:86:00.0.

If some operator messes up and provides two types for the same pGPU, an InvalidLibvirtGPUConfig exception will be raised. If the operator forgets to provide a type for a specific pGPU, then the first type given in enabled_vgpu_types will be supported, like the existing situation. If the operator fat-fingers the PCI IDs, then when creating the inventory, it will return an exception.

As one single compute could now support multiple vGPU types, asking operators to provide host aggregates for grouping computes having the same vGPU type becomes irrelevant. Instead, we need to ask operators to amend their flavors for specific GPU capabilities if they care of such things, or Placement will just randomly pick one of the available vGPU types. For this, we propose to standardize GPU capabilities that are unfortunately very vendor specific (eg. a CUDA library version support) by having a nova.virt.vgpu_capabilities module that would translate a vendor-specific vGPU type into a set of os-traits traits. If operators want vendor-specific traits, it’s their responsibility to provide custom traits on the resource providers or ask the community to find a standard trait that would fit their needs.

Alternatives

Instead of creating only one inventory per pGPU, we could try to have children resource providers for a pGPU being GPU types. But then, once an instance would create a vGPU, all the other inventories but the one related to the used type would be having a total=0.

Data model impact

None

REST API impact

None.

Security impact

None.

Notifications impact

None.

Other end user impact

None.

Performance Impact

None.

Other deployer impact

Operators need to either look at the sysfs (for libvirt) for knowing the existing pGPUs and which types are supported.

Developer impact

None.

Upgrade impact

None, as not setting that config option will keep the existing behaviour where we only support the first enabled type across all pGPUs.

Implementation

Assignee(s)

Primary assignee:

bauzas

Other contributors:

None

Work Items

• Create the config option

• Modify the libvirt virt driver code to make use of that option for creating the nested Resource Provider inventories.

Dependencies

None.

Testing

Classic unittests and functional tests.

Documentation Impact

A release note will be added with a ‘feature’ section, and the Virtual GPU documentation will be modified to explain the new feature.

References

None.