Execution Model Overview¶
Xilinx® FPGA based acceleration platform architecture are described in the XRT and Vitis™ Platform Overview document. On Alveo PCIe platforms xocl driver binds to user physical function and xclmgmt driver binds to management physical function. The ioctls exported by xocl are described in XOCL (PCIe User Physical Function) Driver Interfaces document and ioctls exported by xclmgmt are described in XCLMGMT (PCIe Management Physical Function) Driver Interfaces document. On Zynq Ultrascale+ MPSoC platforms zocl driver binds to the accelerator. The ioctls exported by zocl are listed here TODO.
Xilinx® Vitis compiler, v++ compiles user’s device code into xclbin file which contains FPGA bitstream and collection of metadata like memory topology, IP instantiations, etc. xclbin format is defined in Binary Formats document. For Alveo platforms xclmgmt driver provides an ioctl for xclbin download. For Zynq Ultrascale+ MPSoC zocl provides a similar ioctl for xclbin download. Both drivers support FPGA Manager integration. The drivers walk the xclbin sections, program the FPGA fabric, discover the memory topology, initialize the memory managers for the provided memory topology and discover user’s compute units programmed into the FPGA fabric.
Both PCIe based and embedded platforms use a unified multi-thread/process capable memory management API defined in xrt.main document.
For both class of platforms, memory management is performed inside Linux kernel driver. Both drivers use DRM GEM for memory management which includes buffer allocator, buffer mmap support, reference counting of buffers and DMA-BUF export/import. These operations are made available via ioctls exported by the drivers.
Xilinx® PCIe platforms like Alveo PCIe cards support various memory topologies which can be dynamically loaded as part of FPGA image loading step. This means from one FPGA image to another the device may expose one or more memory controllers where each memory controller has its own memory address range. We use Linux drm_mm for allocation of memory and Linux drm_gem framework for mmap handling. Since ordinarily our device memory is not exposed to host CPU (except when we enable PCIe peer-to-peer feature) we use host memory pages to back device memory for mmap support. For syncing between device memory and host memory pages XDMA/QDMA PCIe memory mapped DMA engine is used. Users call sync ioctl to effect DMA in requested direction.
xocl also supports PCIe Host Memory Bridge where it handles pinning of host memory and programming the Address Remapper tables. Section Host Memory Access provides more information.
Xilinx® embedded platforms like Zynq Ultrascale+ MPSoC support various memory topologies as well. In addition to memory shared between PL (FPAG fabric) and PS (ARM A-53) we can also have dedicated memory for PL using a soft memory controller that is instantiated in the PL itself. zocl supports both CMA backed memory management where accelerators in PL use physical addresses and SVM based memory management – with the help of ARM SMMU – where accelerators in PL use virtual addresses also shared with application running on PS.
Both xocl and zocl support structured execution framework. After xclbin has been loaded by the driver compute units defined by the xclbin are live and ready for execution. The compute units are controlled by driver component called Kernel Domain Scheduler (KDS). KDS queues up execution tasks from client processes via ioctls and then schedules them on available compute units. Both drivers export an ioctl for queuing up execution tasks.
User space submits execution commands to KDS in well defined command packets. The commands are defined in Embedded Runtime Library
KDS notifies user process of a submitted execution task completion asynchronously via POSIX poll mechanism. On PCIe platforms KDS leverages hardware scheduler running on Microblaze soft processor for fine control of compute units. Compute units use interrupts to notify xocl/zocl when they are done. KDS also supports polling mode where KDS actively polls the compute units for completion instead of relying on interrupts from compute units.
On PCIe platforms hardware scheduler (referred to above) runs firmware called Embedded Runtime (ERT). ERT receives requests from KDS on hardware out-of-order Command Queue with upto 128 command slots. ERT notifies KDS of work completion by using bits in Status Register and MSI-X interrupts. ERT source code is also included with XRT source on GitHub.
For Alveo boards xclmgmt does the board management like board recovery in case compute units hang the data bus, sensor data collection, AXI Firewall monitoring, clock scaling, power measurement, loading of firmware files on embedded soft processors like ERT and CMC.
A typical user execution flow would like the following:
Load xclbin using DOWNLOAD ioctl
Discover compute unit register map from xclbin
Allocate data buffers to feed to the compute units using CREATE_BO/MAP_BO ioctl calls
Migrate input data buffers from host to device using SYNC_BO ioctl
Allocate an execution command buffer using CREATE_BO/MAP_BO ioctl call and fill the command buffer using data in 2 above and following the format defined in ert.h
Submit the execution command buffer using EXECBUF ioctl
Wait for completion using POSIX poll
Migrate output data buffers from device to host using SYNC_BO ioctl
Release data buffers and command buffer