parisc: improve ptrace support for gdb single-step

Various GCC tests use gdb to simulate a multithreaded application. Many of
these tests have been failing on parisc linux.

GCC does this by using gdb to single-step the application, then gdb is used to
call other test specific code. Where this fails is when the application is
stepped into the delay slot of a taken branch. This sets the PSW B bit. When
the test specific code is executed, this usually clears the PSW B bit.
Currently, gdb is not allowed to set the B bit. So, the code falls through what
should be a taken branch.

The attached patch adds the PSW B bit to the set of bits that gdb is allowed to
set. In order to set the B bit, the trace system call must return using an
interrupt restore. The patch also modifies this code to use the saved IAOQ
values when they are saved by a ptrace syscall or interruption.

Signed-off-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Helge Deller <deller@gmx.de>

arch/parisc/kernel/entry.S

@@ -1865,7 +1865,7 @@ syscall_restore:
 
 	/* Are we being ptraced? */
 	ldw	TASK_FLAGS(%r1),%r19
-	ldi	(_TIF_SINGLESTEP|_TIF_BLOCKSTEP),%r2
+	ldi	_TIF_SYSCALL_TRACE_MASK,%r2
 	and,COND(=)	%r19,%r2,%r0
 	b,n	syscall_restore_rfi
 
@@ -1978,15 +1978,23 @@ syscall_restore_rfi:
 	/* sr2 should be set to zero for userspace syscalls */
 	STREG	%r0,TASK_PT_SR2(%r1)
 
-pt_regs_ok:
 	LDREG	TASK_PT_GR31(%r1),%r2
-	depi	3,31,2,%r2			   /* ensure return to user mode. */
-	STREG	%r2,TASK_PT_IAOQ0(%r1)
+	depi	3,31,2,%r2		   /* ensure return to user mode. */
+	STREG   %r2,TASK_PT_IAOQ0(%r1)
 	ldo	4(%r2),%r2
 	STREG	%r2,TASK_PT_IAOQ1(%r1)
+	b	intr_restore
 	copy	%r25,%r16
+
+pt_regs_ok:
+	LDREG	TASK_PT_IAOQ0(%r1),%r2
+	depi	3,31,2,%r2		   /* ensure return to user mode. */
+	STREG	%r2,TASK_PT_IAOQ0(%r1)
+	LDREG	TASK_PT_IAOQ1(%r1),%r2
+	depi	3,31,2,%r2
+	STREG	%r2,TASK_PT_IAOQ1(%r1)
 	b	intr_restore
-	nop
+	copy	%r25,%r16
 
 	.import schedule,code
 syscall_do_resched:

arch/parisc/kernel/ptrace.c

@@ -26,7 +26,7 @@
 #include <asm/asm-offsets.h>
 
 /* PSW bits we allow the debugger to modify */
-#define USER_PSW_BITS	(PSW_N | PSW_V | PSW_CB)
+#define USER_PSW_BITS	(PSW_N | PSW_B | PSW_V | PSW_CB)
 
 /*
  * Called by kernel/ptrace.c when detaching..