[PATCH] Single-stepping emulated instructions

Occasionally the ppc64 kernel emulates a usermode instruction, for
example in the alignment exception handler.  Kumar Gala pointed out
(in the context of the ppc32 kernel) that if the instruction was being
single-stepped, and we end up emulating the instruction, we should
then send the process a SIGTRAP as if it had not been emulated and the
process had then taken a single-step exception.  This patch implements
this for ppc64.
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/ppc64/kernel/traps.c

@@ -466,6 +466,18 @@ SingleStepException(struct pt_regs *regs)
 	_exception(SIGTRAP, &info, regs);	
 }
 
+/*
+ * After we have successfully emulated an instruction, we have to
+ * check if the instruction was being single-stepped, and if so,
+ * pretend we got a single-step exception.  This was pointed out
+ * by Kumar Gala.  -- paulus
+ */
+static inline void emulate_single_step(struct pt_regs *regs)
+{
+	if (regs->msr & MSR_SE)
+		SingleStepException(regs);
+}
+
 static void dummy_perf(struct pt_regs *regs)
 {
 }
@@ -487,10 +499,8 @@ AlignmentException(struct pt_regs *regs)
 	fixed = fix_alignment(regs);
 
 	if (fixed == 1) {
-		if (!user_mode(regs))
-			PPCDBG(PPCDBG_ALIGNFIXUP, "fix alignment at %lx\n",
-			       regs->nip);
 		regs->nip += 4;	/* skip over emulated instruction */
+		emulate_single_step(regs);
 		return;
 	}