kgdb: enable arch to support XML packet.

The XML packet could be supported by required architecture if the
architecture defines CONFIG_HAVE_ARCH_KGDB_QXFER_PKT and implement its own
kgdb_arch_handle_qxfer_pkt(). Except for the kgdb_arch_handle_qxfer_pkt(),
the architecture also needs to record the feature supported by gdb stub
into the kgdb_arch_gdb_stub_feature, and these features will be reported
to host gdb when gdb stub receives the qSupported packet.

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

include/linux/kgdb.h

@@ -176,6 +176,17 @@ kgdb_arch_handle_exception(int vector, int signo, int err_code,
 			   char *remcom_out_buffer,
 			   struct pt_regs *regs);
 
+/**
+ *	kgdb_arch_handle_qxfer_pkt - Handle architecture specific GDB XML
+ *				     packets.
+ *	@remcom_in_buffer: The buffer of the packet we have read.
+ *	@remcom_out_buffer: The buffer of %BUFMAX bytes to write a packet into.
+ */
+
+extern void
+kgdb_arch_handle_qxfer_pkt(char *remcom_in_buffer,
+			   char *remcom_out_buffer);
+
 /**
  *	kgdb_call_nmi_hook - Call kgdb_nmicallback() on the current CPU
  *	@ignored: This parameter is only here to match the prototype.

kernel/debug/gdbstub.c

@@ -792,6 +792,19 @@ static void gdb_cmd_query(struct kgdb_state *ks)
 		}
 		break;
 #endif
+#ifdef CONFIG_HAVE_ARCH_KGDB_QXFER_PKT
+	case 'S':
+		if (!strncmp(remcom_in_buffer, "qSupported:", 11))
+			strcpy(remcom_out_buffer, kgdb_arch_gdb_stub_feature);
+		break;
+	case 'X':
+		if (!strncmp(remcom_in_buffer, "qXfer:", 6))
+			kgdb_arch_handle_qxfer_pkt(remcom_in_buffer,
+						   remcom_out_buffer);
+		break;
+#endif
+	default:
+		break;
 	}
 }
 

lib/Kconfig.kgdb

@@ -3,6 +3,11 @@
 config HAVE_ARCH_KGDB
 	bool
 
+# set if architecture has the its kgdb_arch_handle_qxfer_pkt
+# function to enable gdb stub to address XML packet sent from GDB.
+config HAVE_ARCH_KGDB_QXFER_PKT
+	bool
+
 menuconfig KGDB
 	bool "KGDB: kernel debugger"
 	depends on HAVE_ARCH_KGDB