[dpdk-dev] [PATCH v8 0/5] net/softnic: sw fall-back pmd for traffic mgmt and others

Ferruh Yigit ferruh.yigit at intel.com
Tue Oct 10 20:31:23 CEST 2017

On 10/10/2017 11:18 AM, Jasvinder Singh wrote:
> The SoftNIC PMD is intended to provide SW fall-back options for specific
> ethdev APIs in a generic way to the NICs not supporting those features.
> Currently, the only implemented ethdev API is Traffic Management (TM),
> but other ethdev APIs such as rte_flow, traffic metering & policing, etc
> can be easily implemented.
> Overview:
> * Generic: The SoftNIC PMD works with any "hard" PMD that implements the
>   ethdev API. It does not change the "hard" PMD in any way.
> * Creation: For any given "hard" ethdev port, the user can decide to
>   create an associated "soft" ethdev port to drive the "hard" port. The
>   "soft" port is a virtual device that can be created at app start-up
>   through EAL vdev arg or later through the virtual device API.
> * Configuration: The app explicitly decides which features are to be
>   enabled on the "soft" port and which features are still to be used from
>   the "hard" port. The app continues to explicitly configure both the
>   "hard" and the "soft" ports after the creation of the "soft" port.
> * RX/TX: The app reads packets from/writes packets to the "soft" port
>   instead of the "hard" port. The RX and TX queues of the "soft" port are
>   thread safe, as any ethdev.
> * Execution: The "soft" port is a feature-rich NIC implemented by the CPU,
>   so the run function of the "soft" port has to be executed by the CPU in
>   order to get packets moving between "hard" port and the app.
> * Meets the NFV vision: The app should be (almost) agnostic about the NIC
>   implementation (different vendors/models, HW-SW mix), the app should not
>   require changes to use different NICs, the app should use the same API
>   for all NICs. If a NIC does not implement a specific feature, the HW
>   should be augmented with SW to meet the functionality while still
>   preserving the same API.
> Traffic Management SW fall-back overview:
> * Implements the ethdev traffic management API (rte_tm.h).
> * Based on the existing librte_sched DPDK library.
> Example: Create "soft" port for "hard" port "0000:04:00.1", enable the TM
> feature with default settings:
>           --vdev 'net_softnic0,hard_name=0000:04:00.1,soft_tm=on' 
> Q1: Why generic name, if only TM is supported (for now)?
> A1: The intention is to have SoftNIC PMD implement many other (all?)
>     ethdev APIs under a single "ideal" ethdev, hence the generic name.
>     The initial motivation is TM API, but the mechanism is generic and can
>     be used for many other ethdev APIs. Somebody looking to provide SW
>     fall-back for other ethdev API is likely to end up inventing the same,
>     hence it would be good to consolidate all under a single PMD and have
>     the user explicitly enable/disable the features it needs for each
>     "soft" device.
> Q2: Are there any performance requirements for SoftNIC?
> A2: Yes, performance should be great/decent for every feature, otherwise
>     the SW fall-back is unusable, thus useless.
> Q3: Why not change the "hard" device (and keep a single device) instead of
>     creating a new "soft" device (and thus having two devices)?
> A3: This is not possible with the current librte_ether ethdev
>     implementation. The ethdev->dev_ops are defined as constant structure,
>     so it cannot be changed per device (nor per PMD). The new ops also
>     need memory space to store their context data structures, which
>     requires updating the ethdev->data->dev_private of the existing
>     device; at best, maybe a resize of ethdev->data->dev_private could be
>     done, assuming that librte_ether will introduce a way to find out its
>     size, but this cannot be done while device is running. Other side
>     effects might exist, as the changes are very intrusive, plus it likely
>     needs more changes in librte_ether.
> Q4: Why not call the SW fall-back dev_ops directly in librte_ether for
>     devices which do not support the specific feature? If the device
>     supports the capability, let's call its dev_ops, otherwise call the
>     SW fall-back dev_ops.
> A4: First, similar reasons to Q&A3. This fixes the need to change
>     ethdev->dev_ops of the device, but it does not do anything to fix the
>     other significant issue of where to store the context data structures
>     needed by the SW fall-back functions (which, in this approach, are
>     called implicitly by librte_ether).
>     Second, the SW fall-back options should not be restricted arbitrarily
>     by the librte_ether library, the decision should belong to the app.
>     For example, the TM SW fall-back should not be limited to only
>     librte_sched, which (like any SW fall-back) is limited to a specific
>     hierarchy and feature set, it cannot do any possible hierarchy. If
>     alternatives exist, the one to use should be picked by the app, not by
>     the ethdev layer.
> Q5: Why is the app required to continue to configure both the "hard" and
>     the "soft" devices even after the "soft" device has been created? Why
>     not hiding the "hard" device under the "soft" device and have the
>     "soft" device configure the "hard" device under the hood?
> A5: This was the approach tried in the V2 of this patch set (overlay
>     "soft" device taking over the configuration of the underlay "hard"
>     device) and eventually dropped due to increased complexity of having
>     to keep the configuration of two distinct devices in sync with
>     librte_ether implementation that is not friendly towards such
>     approach. Basically, each ethdev API call for the overlay device
>     needs to configure the overlay device, invoke the same configuration
>     with possibly modified parameters for the underlay device, then resume
>     the configuration of overlay device, turning this into a device
>     emulation project.
>     V2 minuses: increased complexity (deal with two devices at same time);
>     need to implement every ethdev API, even those not needed for the scope
>     of SW fall-back; intrusive; sometimes have to silently take decisions
>     that should be left to the app.
>     V3 pluses: lower complexity (only one device); only need to implement
>     those APIs that are in scope of the SW fall-back; non-intrusive (deal
>     with "hard" device through ethdev API); app decisions taken by the app
>     in an explicit way.
> Q6: Why expose the SW fall-back in a PMD and not in a SW library?
> A6: The SW fall-back for an ethdev API has to implement that specific
>     ethdev API, (hence expose an ethdev object through a PMD), as opposed
>     to providing a different API. This approach allows the app to use the
>     same API (NFV vision). For example, we already have a library for TM
>     SW fall-back (librte_sched) that can be called directly by the apps
>     that need to call it outside of ethdev context (use-cases exist), but
>     an app that works with TM-aware NICs through the ethdev TM API would
>     have to be changed significantly in order to work with different
>     TM-agnostic NICs through the librte_sched API.
> Q7: Why have all the SW fall-backs in a single PMD? Why not develop
>     the SW fall-back for each different ethdev API in a separate PMD, then
>     create a chain of "soft" devices for each "hard" device? Potentially,
>     this results in smaller size PMDs that are easier to maintain.
> A7: Arguments for single ethdev/PMD and against chain of ethdevs/PMDs:
>     1. All the existing PMDs for HW NICs implement a lot of features under
>        the same PMD, so there is no reason for single PMD approach to break
>        code modularity. See the V3 code, a lot of care has been taken for
>        code modularity.
>     2. We should avoid the proliferation of SW PMDs.
>     3. A single device should be handled by a single PMD.
>     4. People are used with feature-rich PMDs, not with single-feature
>        PMDs, so we change of mindset?
>     5. [Configuration nightmare] A chain of "soft" devices attached to
>        single "hard" device requires the app to be aware that the N "soft"
>        devices in the chain plus the "hard" device refer to the same HW
>        device, and which device should be invoked to configure which
>        feature. Also the length of the chain and functionality of each
>        link is different for each HW device. This breaks the requirement
>        of preserving the same API while working with different NICs (NFV).
>        This most likely results in a configuration nightmare, nobody is
>        going to seriously use this.
>     6. [Feature inter-dependecy] Sometimes different features need to be
>        configured and executed together (e.g. share the same set of
>        resources, are inter-dependent, etc), so it is better and more
>        performant to do them in the same ethdev/PMD.
>     7. [Code duplication] There is a lot of duplication in the
>        configuration code for the chain of ethdevs approach. The ethdev
>        dev_configure, rx_queue_setup, tx_queue_setup API functions have to
>        be implemented per device, and they become meaningless/inconsistent
>        with the chain approach.
>     8. [Data structure duplication] The per device data structures have to
>        be duplicated and read repeatedly for each "soft" ethdev. The
>        ethdev device, dev_private, data, per RX/TX queue data structures
>        have to be replicated per "soft" device. They have to be re-read for
>        each stage, so the same cache misses are now multiplied with the
>        number of stages in the chain.
>     9. [rte_ring proliferation] Thread safety requirements for ethdev
>        RX/TXqueues require an rte_ring to be used for every RX/TX queue
>        of each "soft" ethdev. This rte_ring proliferation unnecessarily
>        increases the memory footprint and lowers performance, especially
>        when each "soft" ethdev ends up on a different CPU core (ping-pong
>        of cache lines).
>     10.[Meta-data proliferation] A chain of ethdevs is likely to result
>        in proliferation of meta-data that has to be passed between the
>        ethdevs (e.g. policing needs the output of flow classification),
>        which results in more cache line ping-pong between cores, hence
>        performance drops.
> Cristian Dumitrescu (4):
> Jasvinder Singh (4):
>   net/softnic: add softnic PMD
>   net/softnic: add traffic management support
>   net/softnic: add TM capabilities ops
>   net/softnic: add TM hierarchy related ops
> Jasvinder Singh (1):
>   app/testpmd: add traffic management forwarding mode

Series applied to dpdk-next-net/master, thanks.

(Was getting same build error from previous version, fixed while
applying please confirm the pushed commit.
Also waiting for testpmd document to squash this set later)

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