LOCAL: target/xtensa: allow tweaking the list of registers sent to GDB

This commit introduces two environment variables which can be used
to adjust the list of registers sent from QEMU to GDB:
* If QEMU_XTENSA_CORE_REGS_ONLY is set, only non-privileged registers
  will be sent to GDB. This behavior is compatible with Espressif
  builds of GDB up to esp-2021r1.
* If QEMU_XTENSA_COUNT_WINDOW_REGS is set, QEMU will send window
  registers (a0-a15) to GDB. Enable this if you don't have a build of
  GDB which considers a0-a15 to be raw registers.

target/xtensa/gdbstub.c

@@ -46,6 +46,12 @@ void xtensa_count_regs(const XtensaConfig *config,
     unsigned i;
     bool count_core_regs = true;
 
+    /* Espressif local: allow changing the behavior here based on QEMU_XTENSA_COUNT_WINDOW_REGS
+     * environment variable.
+     */
+    const char* count_window_regs_env = getenv("QEMU_XTENSA_COUNT_WINDOW_REGS");
+    bool count_window_regs = count_window_regs_env != NULL && strcmp(count_window_regs_env, "0") != 0;
+
     for (i = 0; config->gdb_regmap.reg[i].targno >= 0; ++i) {
         if (config->gdb_regmap.reg[i].type != xtRegisterTypeTieState &&
             config->gdb_regmap.reg[i].type != xtRegisterTypeMapped &&

target/xtensa/helper.c

@@ -185,6 +185,15 @@ static void xtensa_core_class_init(ObjectClass *oc, void *data)
      * in the gdb/xtensa-config.c inside gdb source tree or inside gdb overlay.
      */
     cc->gdb_num_core_regs = config->gdb_regmap.num_regs;
+
+    /* Espressif local: allow changing the behavior here using
+     * QEMU_XTENSA_CORE_REGS_ONLY environment variable, to support different
+     * GDB builds
+     */
+    const char* core_regs_only = getenv("QEMU_XTENSA_CORE_REGS_ONLY");
+    if (core_regs_only != NULL && strcmp(core_regs_only, "0") != 0) {
+        cc->gdb_num_core_regs = config->gdb_regmap.num_core_regs;
+    }
 }
 
 void xtensa_register_core(XtensaConfigList *node)