s390/boot: Fix ESSA detection

commit c3a589fd9fcbf295a7402a4b188dc9277d505f4f upstream.

The cmma_test_essa() inline assembly uses tmp as input and output, however
tmp is specified as output only, which allows the compiler to optimize the
initialization of tmp away.

Therefore the ESSA detection may or may not work depending on previous
contents of the register that the compiler selected for tmp.

Fix this by using the correct constraint modifier.

Fixes: 468a3bc2b7b9 ("s390/cmma: move parsing of cmma kernel parameter to early boot code")
Cc: stable@vger.kernel.org
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/s390/boot/startup.c b/arch/s390/boot/startup.c
index c2ee0745f59edcf78cf9d75a846d5d6b242d8d12..7b69be63d5d20a932e6451f086c077bea08ddba7 100644 (file)
--- a/arch/s390/boot/startup.c
+++ b/arch/s390/boot/startup.c
@@ -75,7 +75,7 @@ static int cmma_test_essa(void)
                : [reg1] "=&d" (reg1),
                  [reg2] "=&a" (reg2),
                  [rc] "+&d" (rc),
-                 [tmp] "=&d" (tmp),
+                 [tmp] "+&d" (tmp),
                  "+Q" (get_lowcore()->program_new_psw),
                  "=Q" (old)
                : [psw_old] "a" (&old),