SH_EXT_STDOUT_STDERR is a v2.0 semihosting extension: the guest
can open ":tt" with a file mode requesting append access in
order to open stderr, in addition to the existing "open for
read for stdin or write for stdout". Implement this and
report it via the :semihosting-features data.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20190916141544.17540-16-peter.maydell@linaro.org
---
 target/arm/arm-semi.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/target/arm/arm-semi.c b/target/arm/arm-semi.c
index f65d8c907e8..6f7b6d801bf 100644
--- a/target/arm/arm-semi.c
+++ b/target/arm/arm-semi.c
@@ -516,13 +516,14 @@ static uint32_t gdb_flenfn(ARMCPU *cpu, GuestFD *gf)
 
 /* Feature bits reportable in feature byte 0 */
 #define SH_EXT_EXIT_EXTENDED (1 << 0)
+#define SH_EXT_STDOUT_STDERR (1 << 1)
 
 static const uint8_t featurefile_data[] = {
     SHFB_MAGIC_0,
     SHFB_MAGIC_1,
     SHFB_MAGIC_2,
     SHFB_MAGIC_3,
-    SH_EXT_EXIT_EXTENDED, /* Feature byte 0 */
+    SH_EXT_EXIT_EXTENDED | SH_EXT_STDOUT_STDERR, /* Feature byte 0 */
 };
 
 static void init_featurefile_guestfd(int guestfd)
@@ -710,7 +711,21 @@ target_ulong do_arm_semihosting(CPUARMState *env)
         }
 
         if (strcmp(s, ":tt") == 0) {
-            int result_fileno = arg1 < 4 ? STDIN_FILENO : STDOUT_FILENO;
+            int result_fileno;
+
+            /*
+             * We implement SH_EXT_STDOUT_STDERR, so:
+             *  open for read == stdin
+             *  open for write == stdout
+             *  open for append == stderr
+             */
+            if (arg1 < 4) {
+                result_fileno = STDIN_FILENO;
+            } else if (arg1 < 8) {
+                result_fileno = STDOUT_FILENO;
+            } else {
+                result_fileno = STDERR_FILENO;
+            }
             associate_guestfd(guestfd, result_fileno);
             unlock_user(s, arg0, 0);
             return guestfd;
-- 
2.20.1