mem_ap: fix GDB connections

After commit d9b2607 ("gdb_server: support sparse register
maps"), GDB crashes while requesting the value of 'cpsr' because
the fake register is tagged as not existing.

Change the logic and set all register as existing, while still
limiting the list for the initial GDB request at connect.

Change-Id: I1c4e274c06147683db2a59a8920ae5ccd863e15c
Signed-off-by: Antonio Borneo <[email protected]>
Reviewed-on: https://review.openocd.org/c/openocd/+/8163
Tested-by: jenkins

Author: borneoa

src/target/mem_ap.c

@@ -194,11 +194,11 @@ static const char *mem_ap_get_gdb_arch(const struct target *target)
  * reg[24]:     32 bits, fps
  * reg[25]:     32 bits, cpsr
  *
- * Set 'exist' only to reg[0..15], so initial response to GDB is correct
+ * GDB requires only reg[0..15]
  */
 #define NUM_REGS     26
+#define NUM_GDB_REGS 16
 #define MAX_REG_SIZE 96
-#define REG_EXIST(n) ((n) < 16)
 #define REG_SIZE(n)  ((((n) >= 16) && ((n) < 24)) ? 96 : 32)
 
 struct mem_ap_alloc_reg_list {
@@ -218,14 +218,14 @@ static int mem_ap_get_gdb_reg_list(struct target *target, struct reg **reg_list[
 	}
 
 	*reg_list = mem_ap_alloc->reg_list;
-	*reg_list_size = NUM_REGS;
+	*reg_list_size = (reg_class == REG_CLASS_ALL) ? NUM_REGS : NUM_GDB_REGS;
 	struct reg *regs = mem_ap_alloc->regs;
 
 	for (int i = 0; i < NUM_REGS; i++) {
 		regs[i].number = i;
 		regs[i].value = mem_ap_alloc->regs_value;
 		regs[i].size = REG_SIZE(i);
-		regs[i].exist = REG_EXIST(i);
+		regs[i].exist = true;
 		regs[i].type = &mem_ap_reg_arch_type;
 		(*reg_list)[i] = &regs[i];
 	}