As an RTL designer, you might be familiar with or have worked with the Vivado Design Suite. This tool is also at the heart of the Vitis design environment, and your experience working with the Vivado tool will benefit you in this design flow.
RTL Development
The RTL kernel development flow requires you to standardize any RTL IP you have for use in the Vitis design flow. This means creating RTL kernel (.xo) files from existing IP, making any necessary modifications to support the AXI4 interfaces required for the Vitis tools as described in Requirements of an RTL Kernel. The RTL kernel development flow lets you modify existing custom IP that you might have already packaged for use in the Vivado tool, or start your RTL design from scratch and package it for use in the Vitis flow.
Creating an RTL kernel follows the tradition RTL IP development process: you will write the RTL code, simulate it with the Vivado logic simulator, or any supported third-party simulator, and place and route the design to insure it passes timing and routes to completion. You might also define XDC constraints for use with your design as needed to complete implementation. When you are ready, or when your design is complete, you can package the RTL design as an IP and generate the RTL kernel (.xo) files for use by the Vitis compiler in building the system design. In terms of the RTL design and IP packaging process everything is the same, with the additional output of the .xo file.
System Design
In the Vivado development flow you would manually add and stitch the required IP together using the IP integrator of the tool, or define your top-down system using RTL. In the Vivado flow you will need to specify the overall system design, complete with PCIe bus, global memory, and peripheral features, outside of the FPGA design. You would need to create the custom host code incorporating drivers to access features of the system card or the programmable logic.
In the Vitis application acceleration flow the compiler links the RTL kernel, or multiple kernels, with the target platform of the Alveo accelerator card, automatically building the system design using the IP integrator feature. The Vitis compiler automatically instantiates a Memory Subsystem (MSS) IP into the system design to manage AXI traffic between kernels, the host processor, and memory resources. Configuration of the MSS is derived from the connectivity section of a configuration file used during linking as described in Linking the Kernels. XRT provides the underlying runtime and drivers, and provides an API for developing the host application to access the accelerator card.
The Vivado development flow requires synthesis, place and route, and timing closure for your design. The Vitis flow creates the Vivado project during the linking process, and automates the synthesis and implementation of the design. While this is automated within the Vitis tool flow, you can completely control the process, using Tcl scripts or working interactively in the Vivado tool to address design problems and generate the desired results.
While the Vivado and Vitis tools both provide the system design capability, the Vitis tools standardize much of the required ecosystem. The Vitis flow automates several steps like integrating with PCIe, and adding global memories. This lets you focus on developing the RTL function and reduces the overall development time. With the Vitis flow, it is also easier to migrate to another accelerator card seamlessly, and most of the time without any change in the RTL component or the host code.