platform: qemu-virt: Implement system shutdown

In order for QEMU to be compatible with Spike, it implements
a simple protocol used for reporting back the simulation's
status, through the memory-mapped "test finisher" device. We
use that protocol to make QEMU exit on system shutdown.

Signed-off-by: Nick Kossifidis <mick@ics.forth.gr>

platform/qemu/virt/platform.c

@@ -5,9 +5,11 @@
  *
  * Authors:
  *   Anup Patel <anup.patel@wdc.com>
+ *   Nick Kossifidis <mick@ics.forth.gr>
  */
 
 #include <sbi/riscv_encoding.h>
+#include <sbi/riscv_io.h>
 #include <sbi/sbi_const.h>
 #include <sbi/sbi_hart.h>
 #include <sbi/sbi_platform.h>
@@ -19,6 +21,8 @@
 #define VIRT_HART_STACK_SIZE		8192
 
 #define VIRT_TEST_ADDR			0x100000
+#define VIRT_TEST_FINISHER_FAIL		0x3333
+#define VIRT_TEST_FINISHER_PASS		0x5555
 
 #define VIRT_CLINT_ADDR			0x2000000
 
@@ -122,7 +126,11 @@ static int virt_timer_init(bool cold_boot)
 
 static int virt_system_down(u32 type)
 {
-	/* For now nothing to do. */
+	/* Tell the "finisher" that the simulation
+	 * was successful so that QEMU exits
+	 */
+	writew(VIRT_TEST_FINISHER_PASS, (void *)VIRT_TEST_ADDR);
+
 	return 0;
 }