Enable passthrough of SR-IOV physical functions to instances¶
Nova has supported passthrough of PCI devices with its libvirt driver for a few releases already, during which time the code has seen some stabilization and a few minor feature additions.
In the case of SR-IOV enabled cards, it is possible to treat any port on the card either as a number of virtual devices (called VFs - virtual functions) or as a full device (PF - physical function).
Nova’s current handling exposes only virtual functions as resources that can be requested by instances - and this is the most common use case by far. However with the rise of the requirements to virtualize network applications, it can be necessary to give instances full control over the port and not just a single virtual function.
OpenStack is seen as one of the central bits of technology for the NFV use-cases, and a lot of the work has already gone into making OpenStack and Nova NFV enabled, so we want to make sure that we close these small remaining gaps.
Currently it is not possible to pass through a physical function to an OpenStack instance, but some NFV applications need to have full control of the port, while others are happy with using a VF of an SR-IOV enabled card. It is beneficial to be able to do so with the same set of cards, as pre-provisioning resources on the granularity smaller than compute hosts is cumbersome to manage and goes against the goal of Nova to provide on demand resources. We want to be able to give certain instances unlimited access to the port by assigning the PF to it, but revert back to using VFs when the PF is not being used, so as to ensure on-demand provisioning of available resources. This may not be possible with every SR-IOV card and their respective Linux drivers, in which case certain ports will need to be pre-provisioned as either PFs or VFs by administratior ahead of time.
This in turn means that Nova would have to keep track of which VFs belong to particular PFs and make sure that this is reflected in the way resources are tracked (so even a single VF being used means the related PF is unavailable and vice versa, if a PF is being used, all of it’s VFs are marked as used).
PCI device management code in Nova currently filters out any device that is a physical function (this is currently hard-coded). In addition, modeling of PCI device resources in Nova currently assumes flat hierarchy and resource tracking logic does not understand the relationship between different PCI devices that can be exposed to Nova.
Certain NFV workloads may need to have the full control of the physical device, in order to use some of the functionality not available to VFs, to bypass some limitations certian cards impose on VFs, or to exclusively use the full bandwidth of the port. However, due to the dynamic nature of the elastic cloud, and the promise of Nova to deliver resources on demand, we do not wish to have to pre-provision certain SR-IOV cards to be used as PFs as this defeats the promise of the infrastructure management tool that allows for quick re-purposing of resources that Nova brings.
Modern SR-IOV enabled cards along with their drivers usually allow for such reconfiguration to be done on the fly, so once the passthrough of the PF is no longer needed on a specific host (either the instance using it got moved or deleted), the PF is bound back to it’s Linux driver, thus enabling the use of VFs provided that initialization steps (if any are needed) are done upon handing the device back. It is not possible to guarantee that this always works however, due to the vast range of equipment and drivers available on the market, so we want to make sure that there is a way to tell Nova that a card is in certain configuration and cannot be assumed to be reconfigurable.
Additional use cases (that will require further work) will be enabled by having the Nova data model usefully express the relationship between PF and its VFs. Some of them have been proposed as separate specs (see  and ).
Two problems we need to solve are:
How to enable requesting a full physical device. This means extending the InstancePCIRequest data model to be able to hold this information. Since the whitelist parsing logic that builds up the Spec objects probes the system and has the information about whether a device is a PF or not, it is enough to add a physical_function field to the PCI alias schema and the PCIRequest object.
Enable scheduling and resource tracking based on the request that can now be for the whole device. This means extending the data model for PCIDevices to hold information about relationship between physical and virtual functions (this relationship is already recorded but not in a suitable format), and also extending the way we expose the aggregate data about PCI devices to the resource tracker (a.k.a. the PCIDeviceStats class) to be able to present PFs and their counts, and to make sure to track the corresponding VFs that become unavailable once the PF is claimed/used.
In addition to the above, we will want to make sure that whitelist syntax can support passing throught PFs. This will require very few changes it turns out. Currently if a whitelist entry specifies an address or a devname of a PF, the matching code will make sure any of the VFs match. This behavior, combined with allowing a device that is a PF to be tracked by nova (by removing the hard-coded check that skips any PFs) should be sufficient to allow most of the flexibility administrators need. As it is not sufficient for a device to be whitelisted to be requestable by users (it needs to either have an alias that is specified on the flavor), simply defaulting to whitelisting PFs along with all of their VFs if a PF address is whitelisted gives us the flexibility we need, while keeping backwards compatibility.
As is the case with the current implementation, there is some initial configuration that will be needed on hosts that have PCI devices that can be passed through. In addition to the standard setup needed to enable SR-IOV and configure the cards, and whitelist configuration setup that Nova requires, administrators may also need to add an automated way (such as udev rules) to re-enable VFs, since depending on the driver and the card used, any existing configuration may be lost once a VM is given full control of the port, and the device is unbound from the host driver.
In order for PFs to work as Neutron ports, some additional work that is outside of scope of this blueprint will be needed. We aim to make internal Nova changes that are needed the focus here and defer on the integration work to a future (possibly cross-project) blueprint. For the libvirt driver, this means that, since there will be no Neutron support at first, the only way to assign such a device would be using the <hostdev> element, and no support for <interface> is in scope for this blueprint.
There are no real alternatives that cover all of the use cases. An alternative that would cover only the requirement for bandwidth would be to allow for reserving of all VFs of a single PF by a single instance while using only a single VF, effectively reserving the bandwidth. In addition to not being a solution for all the applications, it also does not reduce the complexity of the change much as the relationship between VFs still needs to be modeled in Nova.
Data model impact¶
Even though there is a way currently to figure out the PF a single VF belongs to (through the use of extra_info free-form field) it may be necessary to add a more “query friendly” relationship, that will allow us to answer the question “given a PCI device record that is a PF, which VF records does it contain”.
It is likely to be implemented as a foreign key relationship to the same table, and objects support will be added, but the actual implementation discussion is better suited for the actual code proposal review.
It will also be necessary to be able to know relations between individual PFs and VFs in the aggregate view of the PCI device data used in scheduling, so changes to the way PciDeviceStats holds aggregate data. This will also result in changes to the filtering/claiming logic, the extent of which may impact decisions about the data model so this is best discussed on actual implementation changes.
REST API impact¶
There are no API changes required. PCI devices are requested through flavor extra-specs by specifying an alias of a device specification. Currently, device specifications and their aliases are part of the Nova deployment configuration, and thus are deployment specific.
Other end user impact¶
None - non-admin users will continue to use only things exposed to them via flavor extra-specs, which they cannot modify in any way.
Scheduling of instances requiring PCI passthrough devices will be doing more work and on a bit more data than currently in the case of PF requests. It is unlikely that this will have any noticeable performance impact however.
Other deployer impact¶
PCI alias syntax for enabling the PCI devices will become more feature-full, in order to account for specifically requesting a PF.
Re-work the DB models and corresponding objects to have explicit relationship between the PF entry and it’s corresponding VFs. Update the claiming logic inside the PCI manager class so that claiming/assigning the PF claims all of it’s VFs and vice versa.
Change the PCIDeviceStats class to expose PFs in it’s pools, and change the claiming/consuming logic to claim appropriate amounts of VFs when a PF is consumed or claimed. Once this work item is complete, all of the scheduling and resource tracking logic will be aware of the PF constraint.
Add support for specifying the PF requirement through the pci_alias configuration options, so that it can be requested through flavor extra-specs.
Changes proposed here only extend existing functionality, so they will require updating the current test suite to make sure new functionality is covered. It is expected that the tests currently in place are to prevent any regression to the existing functionality. No new test suites are required to be added for this functionality, only new test cases.
Documentation for the PCI passthrough features in Nova will need to be updated to reflect the above changes - that is to say - no impact out of the ordinary.
Optional section for Mitaka intended to be used each time the spec is updated to describe new design, API or any database schema updated. Useful to let reader understand what’s happened along the time.