doc/accel_fw: Add accel_fw docs and patch series changelog

## v20.07: (Upcoming Release)

### accel_fw

The accel_fw was updated to support compare, dualcast, crc32c.

The accel_fw introduced batching support for all commands in all plug-ins.

See docs for detailed information.

### idxd

The idxd library and plug-in module for the accel_fw were updated to support

all accel_fw commands as well as batching.  Batching is supported both

through the library and the plug-in module.

### ioat

A new API `spdk_ioat_get_max_descriptors` was added.

# All remaining pages are listed here in alphabetical order by filename.

INPUT                  += \

                         about.md \

                         accel_fw.md \

                         applications.md \

                         bdev.md \

                         bdev_module.md \

# Acceleration Framework {#accel_fw}

SPDK provides a framework for abstracting general acceleration capabilities

that can be implemented through plug-in modules and low-level libraries. These

plug-in modules include support for hardware acceleration engines such as

the Intel(R) I/O Acceleration Technology (IOAT) engine and the Intel(R) Data

Streaming Accelerator (DSA) engine. Additionally, a software plug-in module

exists to enable use of the framework in environments without hardware

acceleration capabilities. ISA/L is used for optimized CRC32C calculation within

the software module.

The framework includes an API for getting the current capabilities of the

selected module. See [`spdk_accel_get_capabilities`](https://spdk.io/doc/accel__engine_8h.html) for more details. For the software module, all capabilities will be reported as supported. For the hardware modules, only functions accelerated by hardware will be reported however any function can still be called, it will just be backed by software if it is not reported as a supported capability.

# Acceleration Framework Functions {#accel_functions}

Functions implemented via the framework can be found in the DoxyGen documentation of the

framework public header file here [accel_engine.h](https://spdk.io/doc/accel__engine_8h.html)

# Acceleration Framework Design Considerations {#accel_dc}

The general interface is defined by `/include/accel_engine.h` and implemented

in `/lib/accel`.  These functions may be called by an SPDK application and in

most cases, except where otherwise documented, are asynchronous and follow the

standard SPDK model for callbacks with a callback argument.

If the acceleration framework is started without initializing a hardware module,

optimized software implementations of the functions will back the public API.

Additionally, if any hardware module does not support a specific function and that

hardware module is initialized, the specific function will fallback to a software

optimized implementation.  For example, IOAT does not support the dualcast function

in hardware but if the IOAT module has been initialized and the public dualcast API

is called, it will actually be done via software behind the scenes.

# Acceleration Low Level Libraries {#accel_libs}

Low level libraries provide only the most basic functions that are specific to

the hardware. Low level libraries are located in the '/lib' directory with the

exception of the software implementation which is implemented as part of the

framework itself. The software low level library does not expose a public API.

Applications may choose to interact directly with a low level library if there are

specific needs/considerations not met via accessing the library through the

framework/module. Note that when using the low level libraries directly, the

framework abstracted interface is bypassed as the application will call the public

functions exposed by the individual low level libraries. Thus, code written this

way needs to be certain that the underlying hardware exists everywhere that it runs.

The low level library for IOAT is located in `/lib/ioat`.  The low level library

for DSA is in `/liv/idxd` (IDXD stands for Intel(R) Data Acceleration Driver).

# Acceleration Plug-In Modules {#accel_modules}

Plug-in modules depend on low level libraries to interact with the hardware and

add additional functionality such as queueing during busy conditions or flow

control in some cases. The framework in turn depends on the modules to provide

the complete implementation of the acceleration component. A module must be

selected via startup RPC when the application is started. Otherwise, if no startup

RPC is provided, the framework is available and will use the software plug-in module.

## IOAT Module {#accel_ioat}

To use the IOAT engine, use the RPC [`ioat_scan_accel_engine`](https://spdk.io/doc/jsonrpc.html) before starting the application.

## IDXD Module {#accel_idxd}

To use the DSA engine, use the RPC [`idxd_scan_accel_engine`](https://spdk.io/doc/jsonrpc.html) with an optional parameter of `-c` and provide a configuration number of either 0 or 1. These pre-defined configurations determine how the DSA engine will be setup in terms

of work queues and engines.  The DSA engine is very flexible allowing for various configurations of these elements to either account for different quality of service requirements or to isolate hardware paths where the back end media is of varying latency (i.e. persistent memory vs DRAM).  The pre-defined configurations are as follows:

0: Four separate work queues each backed with one DSA engine.  This is a generic

configuration that provides 4 portals to submit operations to each with a

single engine behind it providing some level of isolation as operations are

submitted round-robin.

1: Two separate work queues each backed with two DSA engines.  This is another

generic configuration that provides 2 portals to submit operations to and

lets the DSA hardware decide which engine to select based on loading.

There are several other configurations that are possible that include quality

of service parameters on the work queues that are not currently utilized by

the module. Specialized use of DSA may require different configurations that

can be added to the module as needed.

## Software Module {#accel_sw}

The software module is enabled by default. If no hardware engine is explicitly

enabled via startup RPC as discussed earlier, the software module will use ISA-L

if available for functions such as CRC32C. Otherwise, standard glibc calls are

used to back the framework API.

## Batching {#batching}

Batching is exposed by the acceleration framework and provides an interface to

batch sets of commands up and then submit them with a single command.  The public

API is consistent with the implementation however each plug-in module behaves

differently depending on its capabilities.

The DSA engine has complete support for batching all supported commands together

into one submission. This is advantageous as it reduces the overhead incurred in

the submission process to the hardware.

The software engine supports batching only to be consistent with the framework API.

In software there is no savings by batching sets of commands versus submitting them

individually.

The IOAT engine supports batching but it is only beneficial for `memmove` and `memfill`

as these are supported by the hardware.  All other commands can be batched and the

framework will manage all other commands via software.

- @subpage event

- @subpage logical_volumes

- @subpage vpp_integration

- @subpage accel_fw