======================================== Attaching physical PCI devices to guests ======================================== The PCI passthrough feature in OpenStack allows full access and direct control of a physical PCI device in guests. This mechanism is generic for any kind of PCI device, and runs with a Network Interface Card (NIC), Graphics Processing Unit (GPU), or any other devices that can be attached to a PCI bus. Correct driver installation is the only requirement for the guest to properly use the devices. Some PCI devices provide Single Root I/O Virtualization and Sharing (SR-IOV) capabilities. When SR-IOV is used, a physical device is virtualized and appears as multiple PCI devices. Virtual PCI devices are assigned to the same or different guests. In the case of PCI passthrough, the full physical device is assigned to only one guest and cannot be shared. PCI devices are requested through flavor extra specs, specifically via the :nova:extra-spec:`pci_passthrough:alias` flavor extra spec. This guide demonstrates how to enable PCI passthrough for a type of PCI device with a vendor ID of ``8086`` and a product ID of ``154d`` - an Intel X520 Network Adapter - by mapping them to the alias ``a1``. You should adjust the instructions for other devices with potentially different capabilities. .. note:: For information on creating servers with SR-IOV network interfaces, refer to the :neutron-doc:`Networking Guide `. **Limitations** * Attaching SR-IOV ports to existing servers was not supported until the 22.0.0 Victoria release. Due to various bugs in libvirt and qemu we recommend to use at least libvirt version 6.0.0 and at least qemu version 4.2. * Cold migration (resize) of servers with SR-IOV devices attached was not supported until the 14.0.0 Newton release, see `bug 1512800 `_ for details. .. note:: Nova only supports PCI addresses where the fields are restricted to the following maximum value: * domain - 0xFFFF * bus - 0xFF * slot - 0x1F * function - 0x7 Nova will ignore PCI devices reported by the hypervisor if the address is outside of these ranges. Enabling PCI passthrough ------------------------ Configure compute host ~~~~~~~~~~~~~~~~~~~~~~ To enable PCI passthrough on an x86, Linux-based compute node, the following are required: * VT-d enabled in the BIOS * IOMMU enabled on the host OS, e.g. by adding the ``intel_iommu=on`` or ``amd_iommu=on`` parameter to the kernel parameters * Assignable PCIe devices To enable PCI passthrough on a Hyper-V compute node, the following are required: * Windows 10 or Windows / Hyper-V Server 2016 or newer * VT-d enabled on the host * Assignable PCI devices In order to check the requirements above and if there are any assignable PCI devices, run the following Powershell commands: .. code-block:: console Start-BitsTransfer https://raw.githubusercontent.com/Microsoft/Virtualization-Documentation/master/hyperv-samples/benarm-powershell/DDA/survey-dda.ps1 .\survey-dda.ps1 If the compute node passes all the requirements, the desired assignable PCI devices to be disabled and unmounted from the host, in order to be assignable by Hyper-V. The following can be read for more details: `Hyper-V PCI passthrough`__. .. __: https://devblogs.microsoft.com/scripting/passing-through-devices-to-hyper-v-vms-by-using-discrete-device-assignment/ Configure ``nova-compute`` ~~~~~~~~~~~~~~~~~~~~~~~~~~ Once PCI passthrough has been configured for the host, :program:`nova-compute` must be configured to allow the PCI device to pass through to VMs. This is done using the :oslo.config:option:`pci.device_spec` option. For example, assuming our sample PCI device has a PCI address of ``41:00.0`` on each host: .. code-block:: ini [pci] device_spec = { "address": "0000:41:00.0" } Refer to :oslo.config:option:`pci.device_spec` for syntax information. Alternatively, to enable passthrough of all devices with the same product and vendor ID: .. code-block:: ini [pci] device_spec = { "vendor_id": "8086", "product_id": "154d" } If using vendor and product IDs, all PCI devices matching the ``vendor_id`` and ``product_id`` are added to the pool of PCI devices available for passthrough to VMs. In addition, it is necessary to configure the :oslo.config:option:`pci.alias` option, which is a JSON-style configuration option that allows you to map a given device type, identified by the standard PCI ``vendor_id`` and (optional) ``product_id`` fields, to an arbitrary name or *alias*. This alias can then be used to request a PCI device using the :nova:extra-spec:`pci_passthrough:alias` flavor extra spec, as discussed previously. For our sample device with a vendor ID of ``0x8086`` and a product ID of ``0x154d``, this would be: .. code-block:: ini [pci] alias = { "vendor_id":"8086", "product_id":"154d", "device_type":"type-PF", "name":"a1" } It's important to note the addition of the ``device_type`` field. This is necessary because this PCI device supports SR-IOV. The ``nova-compute`` service categorizes devices into one of three types, depending on the capabilities the devices report: ``type-PF`` The device supports SR-IOV and is the parent or root device. ``type-VF`` The device is a child device of a device that supports SR-IOV. ``type-PCI`` The device does not support SR-IOV. By default, it is only possible to attach ``type-PCI`` devices using PCI passthrough. If you wish to attach ``type-PF`` or ``type-VF`` devices, you must specify the ``device_type`` field in the config option. If the device was a device that did not support SR-IOV, the ``device_type`` field could be omitted. Refer to :oslo.config:option:`pci.alias` for syntax information. .. important:: This option must also be configured on controller nodes. This is discussed later in this document. Once configured, restart the :program:`nova-compute` service. Special Tags ^^^^^^^^^^^^ When specified in :oslo.config:option:`pci.device_spec` some tags have special meaning: ``physical_network`` Associates a device with a physical network label which corresponds to the ``physical_network`` attribute of a network segment object in Neutron. For virtual networks such as overlays a value of ``null`` should be specified as follows: ``"physical_network": null``. In the case of physical networks, this tag is used to supply the metadata necessary for identifying a switched fabric to which a PCI device belongs and associate the port with the correct network segment in the networking backend. Besides typical SR-IOV scenarios, this tag can be used for remote-managed devices in conjunction with the ``remote_managed`` tag. .. note:: The use of ``"physical_network": null`` is only supported in single segment networks. This is due to Nova not supporting multisegment networks for SR-IOV ports. See `bug 1983570 `_ for details. ``remote_managed`` Used to specify whether a PCI device is managed remotely or not. By default, devices are implicitly tagged as ``"remote_managed": "false"`` but and they must be tagged as ``"remote_managed": "true"`` if ports with ``VNIC_TYPE_REMOTE_MANAGED`` are intended to be used. Once that is done, those PCI devices will not be available for allocation for regular PCI passthrough use. Specifying ``"remote_managed": "true"`` is only valid for SR-IOV VFs and specifying it for PFs is prohibited. .. important:: It is recommended that PCI VFs that are meant to be remote-managed (e.g. the ones provided by SmartNIC DPUs) are tagged as remote-managed in order to prevent them from being allocated for regular PCI passthrough since they have to be programmed accordingly at the host that has access to the NIC switch control plane. If this is not done, instances requesting regular SR-IOV ports may get a device that will not be configured correctly and will not be usable for sending network traffic. .. important:: For the Libvirt virt driver, clearing a VLAN by programming VLAN 0 must not result in errors in the VF kernel driver at the compute host. Before v8.1.0 Libvirt clears a VLAN before passing a VF through to the guest which may result in an error depending on your driver and kernel version (see, for example, `this bug `_ which discusses a case relevant to one driver). As of Libvirt v8.1.0, EPERM errors encountered while programming a VLAN are ignored if VLAN clearning is not explicitly requested in the device XML. ``trusted`` If a port is requested to be trusted by specifying an extra option during port creation via ``--binding-profile trusted=true``, only devices tagged as ``trusted: "true"`` will be allocated to instances. Nova will then configure those devices as trusted by the network controller through its PF device driver. The specific set of features allowed by the trusted mode of a VF will differ depending on the network controller itself, its firmware version and what a PF device driver version allows to pass to the NIC. Common features to be affected by this tag are changing the VF MAC address, enabling promiscuous mode or multicast promiscuous mode. .. important:: While the ``trusted tag`` does not directly conflict with the ``remote_managed`` tag, network controllers in SmartNIC DPUs may prohibit setting the ``trusted`` mode on a VF via a PF device driver in the first place. It is recommended to test specific devices, drivers and firmware versions before assuming this feature can be used. Configure ``nova-scheduler`` ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The :program:`nova-scheduler` service must be configured to enable the ``PciPassthroughFilter``. To do this, add this filter to the list of filters specified in :oslo.config:option:`filter_scheduler.enabled_filters` and set :oslo.config:option:`filter_scheduler.available_filters` to the default of ``nova.scheduler.filters.all_filters``. For example: .. code-block:: ini [filter_scheduler] enabled_filters = ...,PciPassthroughFilter available_filters = nova.scheduler.filters.all_filters Once done, restart the :program:`nova-scheduler` service. Configure ``nova-api`` ~~~~~~~~~~~~~~~~~~~~~~ It is necessary to also configure the :oslo.config:option:`pci.alias` config option on the controller. This configuration should match the configuration found on the compute nodes. For example: .. code-block:: ini [pci] alias = { "vendor_id":"8086", "product_id":"154d", "device_type":"type-PF", "name":"a1", "numa_policy":"preferred" } Refer to :oslo.config:option:`pci.alias` for syntax information. Refer to :ref:`Affinity ` for ``numa_policy`` information. Once configured, restart the :program:`nova-api` service. Configuring a flavor or image ----------------------------- Once the alias has been configured, it can be used for an flavor extra spec. For example, to request two of the PCI devices referenced by alias ``a1``, run: .. code-block:: console $ openstack flavor set m1.large --property "pci_passthrough:alias"="a1:2" For more information about the syntax for ``pci_passthrough:alias``, refer to :doc:`the documentation `. .. _pci-numa-affinity-policy: PCI-NUMA affinity policies -------------------------- By default, the libvirt driver enforces strict NUMA affinity for PCI devices, be they PCI passthrough devices or neutron SR-IOV interfaces. This means that by default a PCI device must be allocated from the same host NUMA node as at least one of the instance's CPUs. This isn't always necessary, however, and you can configure this policy using the :nova:extra-spec:`hw:pci_numa_affinity_policy` flavor extra spec or equivalent image metadata property. There are three possible values allowed: **required** This policy means that nova will boot instances with PCI devices **only** if at least one of the NUMA nodes of the instance is associated with these PCI devices. It means that if NUMA node info for some PCI devices could not be determined, those PCI devices wouldn't be consumable by the instance. This provides maximum performance. **socket** This policy means that the PCI device must be affined to the same host socket as at least one of the guest NUMA nodes. For example, consider a system with two sockets, each with two NUMA nodes, numbered node 0 and node 1 on socket 0, and node 2 and node 3 on socket 1. There is a PCI device affined to node 0. An PCI instance with two guest NUMA nodes and the ``socket`` policy can be affined to either: * node 0 and node 1 * node 0 and node 2 * node 0 and node 3 * node 1 and node 2 * node 1 and node 3 The instance cannot be affined to node 2 and node 3, as neither of those are on the same socket as the PCI device. If the other nodes are consumed by other instances and only nodes 2 and 3 are available, the instance will not boot. **preferred** This policy means that ``nova-scheduler`` will choose a compute host with minimal consideration for the NUMA affinity of PCI devices. ``nova-compute`` will attempt a best effort selection of PCI devices based on NUMA affinity, however, if this is not possible then ``nova-compute`` will fall back to scheduling on a NUMA node that is not associated with the PCI device. **legacy** This is the default policy and it describes the current nova behavior. Usually we have information about association of PCI devices with NUMA nodes. However, some PCI devices do not provide such information. The ``legacy`` value will mean that nova will boot instances with PCI device if either: * The PCI device is associated with at least one NUMA nodes on which the instance will be booted * There is no information about PCI-NUMA affinity available For example, to configure a flavor to use the ``preferred`` PCI NUMA affinity policy for any neutron SR-IOV interfaces attached by the user: .. code-block:: console $ openstack flavor set $FLAVOR \ --property hw:pci_numa_affinity_policy=preferred You can also configure this for PCI passthrough devices by specifying the policy in the alias configuration via :oslo.config:option:`pci.alias`. For more information, refer to :oslo.config:option:`the documentation `. PCI tracking in Placement ------------------------- .. note:: The feature described below are optional and disabled by default in nova 26.0.0. (Zed). The legacy PCI tracker code path is still supported and enabled. The Placement PCI tracking can be enabled via the :oslo.config:option:`pci.report_in_placement` configuration. But please note that once it is enabled on a given compute host it cannot be disabled there any more. Since nova 26.0.0 (Zed) PCI passthrough device inventories are tracked in Placement. If a PCI device exists on the hypervisor and matches one of the device specifications configured via :oslo.config:option:`pci.device_spec` then Placement will have a representation of the device. Each PCI device of type ``type-PCI`` and ``type-PF`` will be modeled as a Placement resource provider (RP) with the name ``_``. A devices with type ``type-VF`` is represented by its parent PCI device, the PF, as resource provider. By default nova will use ``CUSTOM_PCI__`` as the resource class in PCI inventories in Placement. However the name of the resource class can be customized via the ``resource_class`` tag in the :oslo.config:option:`pci.device_spec` option. There is also a new ``traits`` tag in that configuration that allows specifying a list of placement traits to be added to the resource provider representing the matching PCI devices. .. note:: In nova 26.0.0 (Zed) the Placement resource tracking of PCI devices does not support SR-IOV devices intended to be consumed via Neutron ports and therefore having ``physical_network`` tag in :oslo.config:option:`pci.device_spec`. Such devices are supported via the legacy PCI tracker code path in Nova. .. note:: Having different resource class or traits configuration for VFs under the same parent PF is not supported and the nova-compute service will refuse to start with such configuration. .. important:: While nova supported configuring both the PF and its children VFs for PCI passthrough in the past, it only allowed consuming either the parent PF or its children VFs. Since 26.0.0. (Zed) the nova-compute service will enforce the same rule for the configuration as well and will refuse to start if both the parent PF and its VFs are configured. .. important:: While nova supported configuring PCI devices by device name via the ``devname`` parameter in :oslo.config:option:`pci.device_spec` in the past, this proved to be problematic as the netdev name of a PCI device could change for multiple reasons during hypervisor reboot. So since nova 26.0.0 (Zed) the nova-compute service will refuse to start with such configuration. It is suggested to use the PCI address of the device instead. The nova-compute service makes sure that already existing instances with PCI allocations in the nova DB will have a corresponding PCI allocation in placement. This allocation healing also acts on any new instances regardless of the status of the scheduling part of this feature to make sure that the nova DB and placement are in sync. There is one limitation of the healing logic. It assumes that there is no in-progress migration when the nova-compute service is upgraded. If there is an in-progress migration, then the PCI allocation on the source host of the migration will not be healed. The placement view will be consistent after such migration is completed or reverted. Reconfiguring the PCI devices on the hypervisor or changing the :oslo.config:option:`pci.device_spec` configuration option and restarting the nova-compute service is supported in the following cases: * new devices are added * devices without allocation is removed Removing a device that has allocations is not supported. If a device having any allocation is removed then the nova-compute service will keep the device and the allocation exists in the nova DB and in placement and logs a warning. If a device with any allocation is reconfigured in a way that an allocated PF is removed and VFs from the same PF is configured (or vice versa) then nova-compute will refuse to start as it would create a situation where both the PF and its VFs are made available for consumption. For deeper technical details please read the `nova specification. `_ Virtual IOMMU support --------------------- With provided :nova:extra-spec:`hw:viommu_model` flavor extra spec or equivalent image metadata property ``hw_viommu_model`` and with the guest CPU architecture and OS allows, we can enable vIOMMU in libvirt driver. .. note:: Enable vIOMMU might introduce significant performance overhead. You can see performance comparision table from `AMD vIOMMU session on KVM Forum 2021`_. For the above reason, vIOMMU should only be enabled for workflow that require it. .. _`AMD vIOMMU session on KVM Forum 2021`: https://static.sched.com/hosted_files/kvmforum2021/da/vIOMMU%20KVM%20Forum%202021%20-%20v4.pdf Here are four possible values allowed for ``hw:viommu_model`` (and ``hw_viommu_model``): **virtio** Supported on Libvirt since 8.3.0, for Q35 and ARM virt guests. **smmuv3** Supported on Libvirt since 5.5.0, for ARM virt guests. **intel** Supported for for Q35 guests. **auto** This option will translate to ``virtio`` if Libvirt supported, else ``intel`` on X86 (Q35) and ``smmuv3`` on AArch64. For the viommu attributes: * ``intremap``, ``caching_mode``, and ``iotlb`` options for viommu (These attributes are driver attributes defined in `Libvirt IOMMU Domain`_) will direcly enabled. * ``eim`` will directly enabled if machine type is Q35. ``eim`` is driver attribute defined in `Libvirt IOMMU Domain`_. .. note:: eim(Extended Interrupt Mode) attribute (with possible values on and off) can be used to configure Extended Interrupt Mode. A q35 domain with split I/O APIC (as described in hypervisor features), and both interrupt remapping and EIM turned on for the IOMMU, will be able to use more than 255 vCPUs. Since 3.4.0 (QEMU/KVM only). * ``aw_bits`` attribute can used to set the address width to allow mapping larger iova addresses in the guest. Since Qemu current supported values are 39 and 48, we directly set this to larger width (48) if Libvirt supported. ``aw_bits`` is driver attribute defined in `Libvirt IOMMU Domain`_. .. _`Libvirt IOMMU Domain`: https://libvirt.org/formatdomain.html#iommu-devices