xtensa: gdbstub: fix backtracing and stack unwinding #90085

dcpleung · 2025-05-16T23:34:14Z

This fixes GDB backtracing by forcibly spilling all registers, and returning 0s for WINDOWSTART and WINDOWBASE. This is effectively telling GDB that none of physical registers contains active data, and must rely on spilled values on stack. Otherwise, GDB will try to look at AR* registers for previous frame(s). Since we don't save all AR* register values, there is nothing for GDB to look at, and thus failing to unwind stack.

With this change, there is no need to populate a portion of the AR* registers, and no need for keep tracking where those registers related to windowed are.

The calculation of number of high registers is not entirely correct. We need to get past the pointer to BSA in the stack frame before reaching the high registers. The location address difference between the BSA and start of high registers then can be used to calculate how many high registers in the stack frame. So correct the start location of high registers in the calculation as it was incorrect before. Though the result would be the same as further divisions would mask this error. However, it is better to correct this for readability. Signed-off-by: Daniel Leung <[email protected]>

This fixes GDB backtracing by forcibly spilling all registers, and returning 0s for WINDOWSTART and WINDOWBASE. This is effectively telling GDB that none of physical registers contains active data, and must rely on spilled values on stack. Otherwise, GDB will try to look at AR* registers for previous frame(s). Since we don't save all AR* register values, there is nothing for GDB to look at, and thus failing to unwind stack. With this change, there is no need to populate a portion of the AR* registers, and no need for keep tracking where those registers related to windowed are. Signed-off-by: Daniel Leung <[email protected]>