microblaze: fix the horror with restarts of sigreturn()

solution a-la arm one - pick a callee-saved register (r30), set it
non-zero when entering a syscall, have sigreturn wrapper zero it out
and pass the value in it to do_notify_resume() as "in_syscall" (actually,
"restarts allowed") argument.

Note that we don't give a damn about ret_from_fork() - return value
is not restart-worthy anyway.

Possible remaining bug: on !MMU we still have _debug_exception()
restartable.  If it hits with -ERESTART_... accidentally in r3, fun happens.
MMU does _not_ have _debug_exception() restartable.  If that's decided to
be a bug (as I strongly suspect it to be), we'll just need to replace
setting r30 to 1 with setting r30 to 0 in !MMU _debug_exception().
Up to microblaze maintainers...

[folded a fix from Michal]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 4 insertions, 1 deletions

diff --git a/arch/microblaze/kernel/entry-nommu.S b/arch/microblaze/kernel/entry-nommu.S
index 29a05d62ec1a..96f97f845495 100644
--- a/arch/microblaze/kernel/entry-nommu.S
+++ b/arch/microblaze/kernel/entry-nommu.S
@@ -280,6 +280,7 @@ ENTRY(_user_exception)
         /* Figure out which function to use for this system call. */
         /* Note Microblaze barrel shift is optional, so don't rely on it */
         add     r12, r12, r12                   /* convert num -> ptr */
+        addik   r30, r0, 1                      /* restarts allowed */
         add     r12, r12, r12
         lwi     r12, r12, sys_call_table        /* Get function pointer */
         addik   r15, r0, ret_to_user-8          /* set return address */
@@ -369,6 +370,7 @@ ENTRY(_debug_exception)
         bralid  r15, send_sig
         add     r7, r0, r0                      /* 3rd param zero */
 
+        addik   r30, r0, 1                      /* restarts allowed ??? */
         /* Restore r3/r4 to work around how ret_to_user works */
         lwi     r3, r1, PT_R3
         lwi     r4, r1, PT_R4
@@ -492,7 +494,7 @@ work_pending:
         nop
 1:      andi    r11, r19, _TIF_SIGPENDING | _TIF_NOTIFY_RESUME
         beqi    r11, no_work_pending
-        addk    r5, r1, r0
+        addk    r5, r30, r0
         bralid  r15, do_notify_resume
         addik   r6, r0, 1
         bri     no_work_pending
@@ -562,6 +564,7 @@ no_work_pending:
         nop
 
 sys_rt_sigreturn_wrapper:
+        addk    r30, r0, r0             /* no restarts for this one */
         brid    sys_rt_sigreturn
         addk    r5, r1, r0