In the Vitis core development kit, an application program is split between a host application and hardware accelerated kernels with a communication channel between them. The host program, written in C/C++ and using API abstractions like OpenCL, runs on a host processor (such as an x86 server or an Arm processor for embedded platforms), while hardware accelerated kernels run within the programmable logic (PL) region of a Xilinx device.
The API calls, managed by XRT, are used to process transactions between the host program and the hardware accelerators. Communication between the host and the kernel, including control and data transfers, occurs across the PCIe® bus or an AXI bus for embedded platforms. While control information is transferred between specific memory locations in the hardware, global memory is used to transfer data between the host program and the kernels. Global memory is accessible by both the host processor and hardware accelerators, while host memory is only accessible by the host application.
For instance, in a typical application, the host first transfers data to be operated on by the kernel from host memory into global memory. The kernel subsequently operates on the data, storing results back to the global memory. Upon kernel completion, the host transfers the results back into the host memory. Data transfers between the host and global memory introduce latency, which can be costly to the overall application. To achieve acceleration in a real system, the benefits achieved by the hardware acceleration kernels must outweigh the added latency of the data transfers.
The target platform contains the FPGA accelerated kernels, global memory, and the direct memory access (DMA) for memory transfers. Kernels can have one or more global memory interfaces and are programmable. The Vitis core development kit execution model can be broken down into the following steps:
- The host program writes the data needed by a kernel into the global memory of the attached device through the PCIe interface on an Alveo Data Center accelerator card, or through the AXI bus on an embedded platform.
- The host program sets up the kernel with its input parameters.
- The host program triggers the execution of the kernel function on the FPGA.
- The kernel performs the required computation while reading data from global memory, as necessary.
- The kernel writes data back to global memory and notifies the host that it has completed its task.
- The host program reads data back from global memory into the host memory and continues processing as needed.
The FPGA can accommodate multiple kernel instances on the accelerator, both different types of kernels, and multiple instances of the same kernel. XRT transparently orchestrates the interactions between the host program and kernels in the accelerator. XRT architecture documentation is available at https://xilinx.github.io/XRT/.