34. Fail-safe poll mode driver library

The Fail-safe poll mode driver library (librte_pmd_failsafe) is a virtual device that allows using any device supporting hotplug (sudden device removal and plugging on its bus), without modifying other components relying on such device (application, other PMDs).

Additionally to the Seamless Hotplug feature, the Fail-safe PMD offers the ability to redirect operations to secondary devices when the primary has been removed from the system.

Note

The library is enabled by default. You can enable it or disable it manually by setting the CONFIG_RTE_LIBRTE_PMD_FAILSAFE configuration option.

34.1. Features

The Fail-safe PMD only supports a limited set of features. If you plan to use a device underneath the Fail-safe PMD with a specific feature, this feature must be supported by the Fail-safe PMD to avoid throwing any error.

A notable exception is the device removal feature. The fail-safe PMD being a virtual device, it cannot currently be removed in the sense of a specific bus hotplug, like for PCI for example. It will however enable this feature for its sub-device automatically, detecting those that are capable and register the relevant callback for such event.

Check the feature matrix for the complete set of supported features.

34.2. Compilation option

This option can be modified in the $RTE_TARGET/build/.config file.

CONFIG_RTE_LIBRTE_PMD_FAILSAFE (default y)

Toggle compiling librte_pmd_failsafe.

34.3. Using the Fail-safe PMD from the EAL command line

The Fail-safe PMD can be used like most other DPDK virtual devices, by passing a --vdev parameter to the EAL when starting the application. The device name must start with the net_failsafe prefix, followed by numbers or letters. This name must be unique for each device. Each fail-safe instance must have at least one sub-device, up to RTE_MAX_ETHPORTS-1.

A sub-device can be any legal DPDK device, including possibly another fail-safe instance.

34.3.1. Fail-safe command line parameters

dev(<iface>) parameter

This parameter allows the user to define a sub-device. The <iface> part of this parameter must be a valid device definition. It could be the argument provided to any -w device specification or the argument that would be given to a --vdev parameter (including a fail-safe). Enclosing the device definition within parenthesis here allows using additional sub-device parameters if need be. They will be passed on to the sub-device.
exec(<shell command>) parameter

This parameter allows the user to provide a command to the fail-safe PMD to execute and define a sub-device. It is done within a regular shell context. The first line of its output is read by the fail-safe PMD and otherwise interpreted as if passed by the regular dev parameter. Any other line is discarded. If the command fail or output an incorrect string, the sub-device is not initialized. All commas within the shell command are replaced by spaces before executing the command. This helps using scripts to specify devices.
mac parameter [MAC address]

This parameter allows the user to set a default MAC address to the fail-safe and all of its sub-devices. If no default mac address is provided, the fail-safe PMD will read the MAC address of the first of its sub-device to be successfully probed and use it as its default MAC address, trying to set it to all of its other sub-devices. If no sub-device was successfully probed at initialization, then a random MAC address is generated, that will be subsequently applied to all sub-device once they are probed.
hotplug_poll parameter [UINT64] (default 2000)

This parameter allows the user to configure the amount of time in milliseconds between two slave upkeep round.

34.3.2. Usage example

This section shows some example of using testpmd with a fail-safe PMD.

To build a PMD and configure DPDK, refer to the document compiling and testing a PMD for a NIC.
Start testpmd. The slave device should be blacklisted from normal EAL operations to avoid probing it twice when in PCI blacklist mode.
```
$RTE_TARGET/build/app/testpmd -c 0xff -n 4 \
   --vdev 'net_failsafe0,mac=de:ad:be:ef:01:02,dev(84:00.0),dev(net_ring0)' \
   -b 84:00.0 -b 00:04.0 -- -i
```
If the slave device being used is not blacklisted, it will be probed by the EAL first. When the fail-safe then tries to initialize it the probe operation fails.

Note that PCI blacklist mode is the default PCI operating mode.

Alternatively, it can be used alongside any other device in whitelist mode.

$RTE_TARGET/build/app/testpmd -c 0xff -n 4 \
   --vdev 'net_failsafe0,mac=de:ad:be:ef:01:02,dev(84:00.0),dev(net_ring0)' \
   -w 81:00.0 -- -i

Start testpmd using a flexible device definition

$RTE_TARGET/build/app/testpmd -c 0xff -n 4 --no-pci \
   --vdev='net_failsafe0,exec(echo 84:00.0)' -- -i

34.4. Using the Fail-safe PMD from an application

This driver strives to be as seamless as possible to existing applications, in order to propose the hotplug functionality in the easiest way possible.

Care must be taken, however, to respect the ether API concerning device access, and in particular, using the RTE_ETH_FOREACH_DEV macro to iterate over ethernet devices, instead of directly accessing them or by writing one’s own device iterator.

34.5. Plug-in feature

A sub-device can be defined without existing on the system when the fail-safe PMD is initialized. Upon probing this device, the fail-safe PMD will detect its absence and postpone its use. It will then register for a periodic check on any missing sub-device.

During this time, the fail-safe PMD can be used normally, configured and told to emit and receive packets. It will store any applied configuration, and try to apply it upon the probing of its missing sub-device. After this configuration pass, the new sub-device will be synchronized with other sub-devices, i.e. be started if the fail-safe PMD has been started by the user before.

34.6. Plug-out feature

A sub-device supporting the device removal event can be removed from its bus at any time. The fail-safe PMD will register a callback for such event and react accordingly. It will try to safely stop, close and uninit the sub-device having emitted this event, allowing it to free its eventual resources.

34.7. Fail-safe glossary

Fallback device: The fail-safe will fail-over onto this device when the preferred device is absent.
Preferred device: The first declared sub-device in the fail-safe parameters. When this device is plugged, it is always used as emitting device. It is the main sub-device and is used as target for configuration operations if there is any ambiguity.
Upkeep round: Periodical process when slaves are serviced. Each devices having a state different to that of the fail-safe device itself, is synchronized with it. Additionally, each slave having the remove flag set are cleaned-up.
Slave: In the context of the fail-safe PMD, synonymous to sub-device.
Sub-device: A device being utilized by the fail-safe PMD. This is another PMD running underneath the fail-safe PMD. Any sub-device can disappear at any time. The fail-safe will ensure that the device removal happens gracefully.