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>

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
 

arch/microblaze/kernel/entry.S

@@ -353,6 +353,7 @@ C_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 */
+	addi	r30, r0, 1			/* restarts allowed */
 	add	r12, r12, r12;
 
 #ifdef DEBUG
@@ -417,7 +418,7 @@ C_ENTRY(ret_from_trap):
 
 	addik	r5, r1, 0;		/* Arg 1: struct pt_regs *regs */
 	bralid	r15, do_notify_resume;	/* Handle any signals */
-	addi	r6, r0, 1;		/* Arg 2: int in_syscall */
+	add	r6, r30, r0;		/* Arg 2: int in_syscall */
 
 /* Finally, return to user state.  */
 1:	set_bip;			/*  Ints masked for state restore */
@@ -464,6 +465,7 @@ C_ENTRY(ret_from_kernel_thread):
 	add	r3, r0, r0
 
 C_ENTRY(sys_rt_sigreturn_wrapper):
+	addik	r30, r0, 0		/* no restarts */
 	brid	sys_rt_sigreturn	/* Do real work */
 	addik	r5, r1, 0;		/* add user context as 1st arg */