The DPC++ Runtime Plugin Interface.

Overview

The DPC++ Runtime Plugin Interface (PI) is an interface layer between the device-agnostic part of DPC++ runtime and the device-specific runtime layers which control execution on devices. It employs the “plugin” mechanism to bind to the device specific runtime layers similar to what is used by libomptarget or OpenCL.

The picture below illustrates the placement of PI within the overall DPC++ runtime stack. Dotted lines show components or paths which are not yet available in the runtime, but are likely to be developed. PI in DPC++ runtime architecture

The plugin interface and the discovery process behind it allows to dynamically plug in implementations based on OpenCL and “native” runtime for a particular device – such as OpenCL for FPGA devices or native runtimes for GPUs. Implementations of the PI are “plugins” - dynamic libraries or shared objects which expose a number of entry points implementing the PI interface. The DPC++ runtime collects those function pointers into a PI interface dispatch table - one per plugin - and uses this table to dispatch to the device(s) covered by the corresponding plugin.

PI is based on a subset of OpenCL 1.2 runtime specification, it follows OpenCL’s platform, execution and memory models in all aspects except for those explicitly mentioned in this document. Some of PI API types and functions have exact matches in OpenCL. Whenever there is such a match, the semantics also fully match unless the differences are explicitly specified in this document. While PI has roots in OpenCL, it does have many differences, and the gap is likely to grow, for example in areas of memory model and management, program management.

Discovery and linkage of PI implementations

PI implementation discovery

Device discovery phase enumerates all available devices and their features by querying underlying plugins found in the system. This process is performed when all attached platforms or devices are queried in an application; for example, during device selection.

Plugin discovery

Plugins are physically dynamic libraries or shared objects. The process to discover plugins follows the following guidelines.

The DPC++ Runtime reads the names of the plugins from a configuration file at a predetermined location (TBD - Add this location). These plugins are searched at locations in env LD_LIBRARY_PATH on Linux and env PATH on Windows. (TBD - Extend to search the plugins at a path relative to the SYCL Runtime installation directory by using DT_RPATH on Linux. Similar functionality can be achieved on Windows using SetDllDirectory. This will help avoiding extra setting of LD_LIBRARY_PATH.) To avoid any issues with read-only access, an environment variable SYCL_PI_CONFIG can be set to point to the configuration file which lists the Plugin names. The enviroment variable if set overrides the predetermined location’s config file. These Plugins are then be searched in LD_LIBRARY_PATH locations. It is the developer’s responsibility to include the plugin names from the predetermined location’s config file to enable discovery of all plugins. (TBD - Extend to support search in DT_RPATH as above.) In the current implementation the plugin names are hardcoded in the library. Configuration file or env SYCL_PI_CONFIG is currently not being considered.

A trace mechanism is provided using env SYCL_PI_TRACE to log the discovery/ binding/ device enumeration process. Different levels of tracing can be achieved with different values of SYCL_PI_TRACE. SYCL_PI_TRACE=0x01 provides basic trace of plugins discovered and bound. It also lists the device selector’s selected device information. SYCL_PI_TRACE=0x02 provides trace of all PI calls made from the DPC++ runtime with arguments and returned values. SYCL_PI_TRACE=-1 lists all PI Traces above and more debug messages.

Plugin binary interface

Plugins should implement all the Interface APIs required for the PI Version it supports. There is pi.def/ pi.h file listing all PI API names that can be called by the specific version of Plugin Interface. It exports a function - “piPluginInit” that returns the plugin details and function pointer table containing the list of pointers to implemented Interface Functions defined in pi.h. In the future, this document will list the minimum set of Interface APIs to be supported by Plugins. This will also require adding functionality to SYCL Runtime to work with such limited functionality plugins.

(TBD - list and describe the symbols that a plugin must implement in order to be picked up by the DPC++ runtime for offload.)

Binding a Plugin

The DPC++ Runtime loads all discovered Plugins and tries to bind them by calling piPluginInit API for each loaded Plugin. The Plugins return the information of supported PI version and the list of implemented PI API Function pointers. (TBD - Use the PI API Version information and check for compatibility. Extend to support version compatibility checks without loading the library. Eg:Changing the plugin name to reflect the supported Plugin Interface version.) The information of compatible plugins (with the Function Pointer Table) is stored in the associated platforms during platform object construction. The PI API calls are later forwarded using this information. A plugin is said to “bind” after this process completes with no errors. During device selection, the user can prefer selection of a device from a specific Plugin or Backend using the env SYCL_BE. The correspondence between a plugin and a SYCL_BE value is currently hardcoded in the runtime. ( TBD: Make this a part of configuration file). Eg: SYCL_BE=PI_OPENCL corresponds to OpenCL Plugin.

OpenCL plugin

OpenCL plugin is a usual plugin from DPC++ runtime standpoint, but its loading and initialization involves a nested discovery process which finds out available OpenCL implementations. They can be installed either in the standard Khronos ICD-compatible way (e.g. listed in files under /etc/OpenCL/vendors on Linux) or not, and the OpenCL plugin can hook up with both.

TBD - implement and describe the nested OpenCL implementation discovery process performed by the OpenCL plugin

Device enumeration by plugins

Devices from all bound plugins are queried and listed as and when required, eg: during device selection in device_selector. The trace shows the PI API calls made when using SYCL_PI_TRACE=-1. (TBD - Add the trace to list all available devices when plugins are successfully bound.)

Plugin Unloading

The plugins not chosen to be connected to should be unloaded. piInitializePlugins() can be called to load and bound the necessary plugins. In addition, piTearDown() can be called when plugins are not needed any more. It notifies each plugin to start performing its own tear-down process such as global memory deallocation. In the future, piTearDown() can include any other jobs that need to be done before the plugin is unloaded from memory. Possibly, a notification of the plugin unloading to lower-level plugins can be added so that they can clean up their own memory [TBD]. After piTearDown() is called, the plugin can be safely unloaded by calling unload(), which is going to invoke OS-specific system calls to remove the dynamic library from memory.

Each plugin should not create global variables that require non-trivial destructor. Pointer variables with heap memory allocation is a good example to be created at the global scope. A std::vector object is not. piTearDown will take care of deallocation of these global variables safely.

PI API Specification

PI interface is logically divided into few subsets:

  • Core API which must be implemented by all plugins for DPC++ runtime to be able to operate on the corresponding device. The core API further breaks down into

    • OpenCL-based APIs which have OpenCL origin and semantics

    • Extension APIs which don’t have counterparts in the OpenCL

  • Interoperability API which allows interoperability with underlying runtimes such as OpenCL.

See pi.h header for the full list and descriptions of PI APIs.

The Core OpenCL-based PI APIs

This subset defines functions representing core functionality, such as device memory management, kernel creation and parameter setting, enqueuing kernel for execution, etc. Functions in this subset fully match semantics of the corresponding OpenCL functions, for example:

piKernelCreate
piKernelRelease
piKernelSetArg

The Extension PI APIs

Those APIs don’t have OpenCL counter parts and require full specification. For example, the function below selects the most appropriate device binary based on runtime information and the binary’s characteristics

pi_result piextDeviceSelectBinary(
  pi_device           device,
  pi_device_binary *  binaries,
  pi_uint32           num_binaries,
  pi_device_binary *  selected_binary);

PI also defines few types and string tags to describe a device binary image. Those are used to communicate to plugins information about the images where it is needed, currently only in the above function. The main type is pi_device_binary, whose detailed description can also be found in the header. The layout of this type strictly matches the layout of the corresponding device binary descriptor type defined in the clang-offload-wrapper tool which wraps device binaries into a host object for further linkage. The wrapped binaries reside inside this descriptor in a data section.

The Interoperability PI APIs

These are APIs needed to implement DPC++ runtime interoperability with underlying “native” device runtimes such as OpenCL. There are some OpenCL interoperability APIs, which are to be implemented by the OpenCL PI plugin only. These APIs match semantics of the corresponding OpenCL APIs exactly. For example:

pi_result piclProgramCreateWithSource(
  pi_context        context,
  pi_uint32         count,
  const char **     strings,
  const size_t *    lengths,
  pi_program *      ret_program);

Some interoperability extension APIs have been added to get native runtime handles from the backend-agnostic PI Objects or to create PI Objects using the native handles. Eg:

pi_result piextDeviceGetNativeHandle(
  pi_device device,
  pi_native_handle *nativeHandle);

pi_result piextDeviceCreateWithNativeHandle(
  pi_native_handle nativeHandle,
  pi_device *device);

PI Extension mechanism

TBD This section describes a mechanism for DPC++ or other runtimes to detect availability of and obtain interfaces beyond those defined by the PI dispatch.

TBD Add API to query PI version supported by plugin at runtime.