P2P FPGA to FPGA Bandwidth Example

This is simple example to explain performance bandwidth for P2P transfer between two FPGA devices.

KEY CONCEPTS: P2P, Multi-FPGA Execution, XDMA

KEYWORDS: XCL_MEM_EXT_P2P_BUFFER

PCIe peer-to-peer communication (P2P) is a PCIe feature which enables two PCIe devices to directly transfer data between each other without using host RAM as a temporary storage. The latest version of Vitis PCIe platforms support P2P feature via PCIe Resizable BAR Capability

In this Example we highlight the data transfer between FPGA and FPGA devices. The current design support P2P between 2 similar FPGA devices. User can update the makefile if need to run P2P between different FPGA cards.

Enable/Disable P2P

XRT xbutil is used to enable/disable P2P feature and check current configuration. P2P configuration is persistent across warm reboot. Enabling or disabling P2P requires root privilege.

Enabling P2P after cold boot is likely to fail because it resizes an existing P2P PCIe BAR to a large size and usually Linux will not reserve large IO memory for the PCIe bridges. A warm reboot is required in this scenario after which BIOS and Linux will reassign the required expanded IO memory resource for P2P BAR. Disabling P2P takes effect immediately. Currently XRT does not check if the P2P memory is in use. Administrator needs to make sure P2P is not in use before disabling it. The result of disabling P2P while it is in use is undefined.

There are three possible values for P2P Enabled field :

Value	Remarks
True	P2P is Enabled
False	P2P is Disabled
no iomem	P2P is enabled in device but system could not allocate IO memory, warm reboot is needed

Enable P2P

Enable P2P after power up sequence.

# xbutil p2p --enable
ERROR: resoure busy, please try warm reboot
# xbutil query
...
PCIe            DMA chan(bidir) MIG Calibrated  P2P Enabled
GEN 3x16        2               true            no iomem
# reboot
...
# xbutil query
...
PCIe            DMA chan(bidir) MIG Calibrated  P2P Enabled
GEN 3x16        2               true            true
...

Disable P2P

# xbutil p2p --disable
# xbutil query
...
PCIe            DMA chan(bidir) MIG Calibrated  P2P Enabled
GEN 3x16        2               true            false
...

EXCLUDED PLATFORMS

Platforms containing following strings in their names are not supported for this example :

zc
201910
2018
vck
aws
u50
_u25_

DESIGN FILES

Application code is located in the src directory. Accelerator binary files will be compiled to the xclbin directory. The xclbin directory is required by the Makefile and its contents will be filled during compilation. A listing of all the files in this example is shown below

src/bandwidth.cpp
src/host.cpp

COMMAND LINE ARGUMENTS

Once the environment has been configured, the application can be executed by

./p2p_fpga2fpga_bandwidth -x1 <bandwidth XCLBIN> -x2 <bandwidth XCLBIN>