[Qemu-ppc] [PATCH v3] 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.
Use SP+16 which is a slot for LR, for save/restore of previous value
of r14. SP+16 can be used, as LR is preserved across the 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
https://openpowerfoundation.org/wp-content/uploads/2016/03/ABI64BitOpenPOWERv1.1_16July2015_pub4.pdf

Signed-off-by: Shivaprasad G Bhat <address@hidden>
---
v2: https://lists.gnu.org/archive/html/qemu-devel/2018-07/msg05102.html
Changes from v2:
   Added code to store and restore r14 register. 

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 |    7 +++++--
 1 file changed, 5 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..ca85da13bd 100644
--- a/linux-user/host/ppc64/safe-syscall.inc.S
+++ b/linux-user/host/ppc64/safe-syscall.inc.S
@@ -49,7 +49,8 @@ safe_syscall_base:
         *               and returns the result in r3
         * Shuffle everything around appropriately.
         */
-       mr      11, 3   /* signal_pending */
+       std     14, 16(1) /* Preserve r14 in SP+16 */
+       mr      14, 3   /* signal_pending */
        mr      0, 4    /* syscall number */
        mr      3, 5    /* syscall arguments */
        mr      4, 6
@@ -67,11 +68,12 @@ 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
 safe_syscall_end:
+       ld 14, 16(1) /* restore r14 to its original value */
        /* code path when we did execute the syscall */
        bnslr+
 
@@ -81,6 +83,7 @@ safe_syscall_end:
 
        /* code path when we didn't execute the syscall */
 0:     addi    3, 0, -TARGET_ERESTARTSYS
+       ld 14, 16(1) /* restore r14 to its orginal value */
        blr
        .cfi_endproc