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xtensa: gdbstub: fix backtracing and stack unwinding

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This fixes GDB backtracing by forcibly spilling all registers,
and faking values for WINDOWSTART and WINDOWBASE. This is
effectively telling GDB that only A0-A3 and AR0-AR3 contain
active data and other physical registers do not. GDB then must
rely on spilled values on stack. Otherwise, GDB will try to
look at all AR* registers for previous frame(s). Since we
do not save all AR* register values, there is nothing for GDB
to look at, and thus failing to unwind stack.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>
This commit is contained in:
Daniel Leung 2025-06-04 10:56:31 -07:00 • committed by Daniel DeGrasse
commit 733305293a
2 changed files with 25 additions and 26 deletions
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48
arch/xtensa/core/gdbstub.c
View file

@ -178,11 +178,20 @@ static void read_sreg(struct gdb_ctx *ctx, struct xtensa_register *reg)
break;
#endif
#if XCHAL_HAVE_WINDOWED
/*
* Returning 0 for WINDOWBASE and 1 for WINDOWSTART. This is
* effectively telling GDB that only A0-A3 and AR0-AR3 contain
* active data and other physical registers do not. GDB then
* must rely on spilled values on stack. Otherwise, GDB will try
* to look at all AR* registers for previous frame(s). Since we
* do not 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);
val = 0;
break;
case WINDOWSTART:
val = get_one_sreg(WINDOWSTART);
val = 1;
break;
#endif
#if XCHAL_NUM_INTLEVELS > 0
@ -426,6 +435,16 @@ 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));
const uint32_t *bsa = *(const int **)stack;
@ -470,26 +489,13 @@ 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;
uint8_t a0_idx, ar_idx;
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.
*/
/* Copied the logical registers A0-A15 to physical registers AR0-AR15. */
for (idx = 0; idx < num_laddr_regs; idx++) {
/* Index to register description array for A */
/* Index to register description array for A and AR */
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;
ar_idx = xtensa_gdb_ctx.ar_idx + 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;
@ -955,10 +961,6 @@ void arch_gdb_init(void)
/* 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;
};

3
include/zephyr/arch/xtensa/gdbstub.h
View file

@ -102,9 +102,6 @@ struct gdb_ctx {
/** Index in register descriptions of AR0 register */
uint8_t ar_idx;
/** Index in register descriptions of WINDOWBASE register */
uint8_t wb_idx;
};
/**