xtensa: gdbstub: fix backtracing and stack unwinding

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]>

Author: dcpleung

arch/xtensa/core/gdbstub.c

@@ -178,11 +178,17 @@ static void read_sreg(struct gdb_ctx *ctx, struct xtensa_register *reg)
 		break;
 #endif
 #if XCHAL_HAVE_WINDOWED
+	/* Returning 0s is needed for stack unwinding to work.
+	 * 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.
+	 */
 	case WINDOWBASE:
-		val = get_one_sreg(WINDOWBASE);
-		break;
+		__fallthrough;
 	case WINDOWSTART:
-		val = get_one_sreg(WINDOWSTART);
+		val = 0;
 		break;
 #endif
 #if XCHAL_NUM_INTLEVELS > 0
@@ -426,6 +432,16 @@ static void copy_to_ctx(struct gdb_ctx *ctx, const struct arch_esf *stack)
 {
 	struct xtensa_register *reg;
 	int idx, num_laddr_regs;
+	int32_t a0save;
+
+	/* Need to spill all registers so their values are on the stack instead of
+	 * the physical register file. This is required for GDB backtracing to
+	 * walk through stack.
+	 */
+	__asm__ volatile("mov %0, a0;"
+			 "call0 xtensa_spill_reg_windows;"
+			 "mov a0, %0"
+			 : "=r"(a0save));
 
 	uint32_t *bsa = *(const int **)stack;
 
@@ -469,33 +485,6 @@ static void copy_to_ctx(struct gdb_ctx *ctx, const struct arch_esf *stack)
 		}
 	}
 
-#if XCHAL_HAVE_WINDOWED
-	uint8_t a0_idx, ar_idx, wb_start;
-
-	wb_start = (uint8_t)xtensa_gdb_ctx.regs[xtensa_gdb_ctx.wb_idx].val;
-
-	/*
-	 * Copied the logical registers A0-A15 to physical registers (AR*)
-	 * according to WINDOWBASE.
-	 */
-	for (idx = 0; idx < num_laddr_regs; idx++) {
-		/* Index to register description array for A */
-		a0_idx = xtensa_gdb_ctx.a0_idx + idx;
-
-		/* Find the start of window (== WINDOWBASE * 4) */
-		ar_idx = wb_start * 4;
-		/* Which logical register we are working on... */
-		ar_idx += idx;
-		/* Wrap around A64 (or A32) -> A0 */
-		ar_idx %= XCHAL_NUM_AREGS;
-		/* Index to register description array for AR */
-		ar_idx += xtensa_gdb_ctx.ar_idx;
-
-		xtensa_gdb_ctx.regs[ar_idx].val = xtensa_gdb_ctx.regs[a0_idx].val;
-		xtensa_gdb_ctx.regs[ar_idx].seqno = xtensa_gdb_ctx.regs[a0_idx].seqno;
-	}
-#endif
-
 	/* Disable stepping */
 	set_one_sreg(ICOUNT, 0);
 	set_one_sreg(ICOUNTLEVEL, 0);
@@ -951,14 +940,6 @@ void arch_gdb_init(void)
 			/* A0: 0x0000 */
 			xtensa_gdb_ctx.a0_idx = idx;
 			break;
-		case XTREG_GRP_ADDR:
-			/* AR0: 0x0100 */
-			xtensa_gdb_ctx.ar_idx = idx;
-			break;
-		case (XTREG_GRP_SPECIAL + WINDOWBASE):
-			/* WINDOWBASE (Special Register) */
-			xtensa_gdb_ctx.wb_idx = idx;
-			break;
 		default:
 			break;
 		};

include/zephyr/arch/xtensa/gdbstub.h

@@ -99,12 +99,6 @@ struct gdb_ctx {
 
 	/** Index in register descriptions of A0 register */
 	uint8_t			a0_idx;
-
-	/** Index in register descriptions of AR0 register */
-	uint8_t			ar_idx;
-
-	/** Index in register descriptions of WINDOWBASE register */
-	uint8_t			wb_idx;
 };
 
 /**