Update difftest/doc/rational.md

Co-authored-by: Jiuyang Liu <[email protected]>

difftest/doc/rational.md

@@ -20,7 +20,7 @@ The goal of offline difftest is to ensure the correctness of each instruction’
    - **Using SAIL Simulator:** SAIL offers a highly formalized and customizable architecture simulation framework. Despite its slower speed, SAIL’s precision and flexibility make it well-suited for offline verification needs.
      - **Removing Platform Dependencies:** To simplify the simulation, platform-related parts of SAIL will be removed, focusing on validating the instruction set and processor core.
      - **Function Patch:** For example, in validating FP (floating-point) related instructions, certain functions, such as `fp reduce order`, may need to be replaced.
-     - **Handling Undefined Behavior:** To verify undefined behavior, an interface will be provided for the simulator to inject specific behavior. This may involve forking the SAIL project or directly customizing its source code.
+     - **Handling Undefined Behavior:** To verify undefined behavior, an interface will be provided for the simulator to inject specific behavior. This may involve adding external functions to SAIL and patching its source code.
 
 ##### 3. **DiffTest Algorithm**
    - **Instruction Alignment:** During the comparison process, it’s crucial to ensure that the DUT and model instruction streams are properly aligned. This may require special alignment algorithms to handle challenges arising from out-of-order or parallel execution.