[Qemu-ppc] [PATCH v2] linux-user: ppc64: don't use volatile register during safe_syscall

[Shivaprasad G Bhat]

r11 is a volatile register on PPC as per calling conventions.
The safe_syscall code uses it to check if the signal_pending
is set during the safe_syscall. When a syscall is interrupted
on return from signal handling, the r11 might be corrupted
before we retry the syscall leading to a crash. The registers
r0-r13 are not to be used here as they have
volatile/designated/reserved usages. Change the code to use
r14 which is non-volatile and is appropriate for local use in
safe_syscall.

Steps to reproduce:
On PPC host, issue `qemu-x86_64 /usr/bin/cc -E -`
Attempt Ctrl-C, the issue is reproduced.

Reference:
https://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi-1.9.html#REG

Signed-off-by: Shivaprasad G Bhat <address@hidden>
Tested-by: Laurent Vivier <address@hidden>
Reviewed-by: Laurent Vivier <address@hidden>
---
v1: https://lists.gnu.org/archive/html/qemu-devel/2018-07/msg05089.html
Changes from v1:
   Fixed the commit message as suggested

 linux-user/host/ppc64/safe-syscall.inc.S |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/linux-user/host/ppc64/safe-syscall.inc.S 
b/linux-user/host/ppc64/safe-syscall.inc.S
index d30050a67c..b0cbbe6a69 100644
--- a/linux-user/host/ppc64/safe-syscall.inc.S
+++ b/linux-user/host/ppc64/safe-syscall.inc.S
@@ -49,7 +49,7 @@ safe_syscall_base:
         *               and returns the result in r3
         * Shuffle everything around appropriately.
         */
-       mr      11, 3   /* signal_pending */
+       mr      14, 3   /* signal_pending */
        mr      0, 4    /* syscall number */
        mr      3, 5    /* syscall arguments */
        mr      4, 6
@@ -67,7 +67,7 @@ safe_syscall_base:
         */
 safe_syscall_start:
        /* if signal_pending is non-zero, don't do the call */
-       lwz     12, 0(11)
+       lwz     12, 0(14)
        cmpwi   0, 12, 0
        bne-    0f
        sc