target/arm/arm-semi: Implement SH_EXT_EXIT_EXTENDED extension

SH_EXT_EXIT_EXTENDED is a v2.0 semihosting extension: it
indicates that the implementation supports the SYS_EXIT_EXTENDED
function. This function allows both A64 and A32/T32 guests to
exit with a specified exit status, unlike the older SYS_EXIT
function which only allowed this for A64 guests. Implement
this extension.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20190916141544.17540-15-peter.maydell@linaro.org

target/arm/arm-semi.c

@@ -59,6 +59,7 @@
 #define TARGET_SYS_HEAPINFO    0x16
 #define TARGET_SYS_EXIT        0x18
 #define TARGET_SYS_SYNCCACHE   0x19
+#define TARGET_SYS_EXIT_EXTENDED 0x20
 
 /* ADP_Stopped_ApplicationExit is used for exit(0),
  * anything else is implemented as exit(1) */
@@ -513,12 +514,15 @@ static uint32_t gdb_flenfn(ARMCPU *cpu, GuestFD *gf)
 #define SHFB_MAGIC_2 0x46
 #define SHFB_MAGIC_3 0x42
 
+/* Feature bits reportable in feature byte 0 */
+#define SH_EXT_EXIT_EXTENDED (1 << 0)
+
 static const uint8_t featurefile_data[] = {
     SHFB_MAGIC_0,
     SHFB_MAGIC_1,
     SHFB_MAGIC_2,
     SHFB_MAGIC_3,
-    0, /* Feature byte 0 */
+    SH_EXT_EXIT_EXTENDED, /* Feature byte 0 */
 };
 
 static void init_featurefile_guestfd(int guestfd)
@@ -1042,11 +1046,14 @@ target_ulong do_arm_semihosting(CPUARMState *env)
             return 0;
         }
     case TARGET_SYS_EXIT:
-        if (is_a64(env)) {
+    case TARGET_SYS_EXIT_EXTENDED:
+        if (nr == TARGET_SYS_EXIT_EXTENDED || is_a64(env)) {
             /*
-             * The A64 version of this call takes a parameter block,
+             * The A64 version of SYS_EXIT takes a parameter block,
              * so the application-exit type can return a subcode which
              * is the exit status code from the application.
+             * SYS_EXIT_EXTENDED is an a new-in-v2.0 optional function
+             * which allows A32/T32 guests to also provide a status code.
              */
             GET_ARG(0);
             GET_ARG(1);
@@ -1058,8 +1065,10 @@ target_ulong do_arm_semihosting(CPUARMState *env)
             }
         } else {
             /*
-             * ARM specifies only Stopped_ApplicationExit as normal
-             * exit, everything else is considered an error
+             * The A32/T32 version of SYS_EXIT specifies only
+             * Stopped_ApplicationExit as normal exit, but does not
+             * allow the guest to specify the exit status code.
+             * Everything else is considered an error.
              */
             ret = (args == ADP_Stopped_ApplicationExit) ? 0 : 1;
         }