powerpc: move norestart trap flag to bit 0

Compact the trap flags down to use the low 4 bits of regs.trap.

A few 64e interrupt trap numbers set bit 4. Although they tended to be
trivial so it wasn't a real problem[1], it is not the right thing to do,
and confusing.

[*] E.g., 0x310 hypercall goes to unknown_exception, which prints
    regs->trap directly so 0x310 will appear fine, and only the syscall
    interrupt will test norestart, so it won't be confused by 0x310.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210316104206.407354-12-npiggin@gmail.com

arch/powerpc/include/asm/ptrace.h

@@ -185,20 +185,25 @@ static inline void regs_set_return_value(struct pt_regs *regs, unsigned long rc)
 #define current_pt_regs() \
 	((struct pt_regs *)((unsigned long)task_stack_page(current) + THREAD_SIZE) - 1)
 
+/*
+ * The 4 low bits (0xf) are available as flags to overload the trap word,
+ * because interrupt vectors have minimum alignment of 0x10. TRAP_FLAGS_MASK
+ * must cover the bits used as flags, including bit 0 which is used as the
+ * "norestart" bit.
+ */
 #ifdef __powerpc64__
-#define TRAP_FLAGS_MASK		0x10
-#define TRAP(regs)		((regs)->trap & ~TRAP_FLAGS_MASK)
+#define TRAP_FLAGS_MASK		0x1
 #else
 /*
  * On 4xx we use bit 1 in the trap word to indicate whether the exception
  * is a critical exception (1 means it is).
  */
-#define TRAP_FLAGS_MASK		0x1E
-#define TRAP(regs)		((regs)->trap & ~TRAP_FLAGS_MASK)
+#define TRAP_FLAGS_MASK		0xf
 #define IS_CRITICAL_EXC(regs)	(((regs)->trap & 2) != 0)
 #define IS_MCHECK_EXC(regs)	(((regs)->trap & 4) != 0)
 #define IS_DEBUG_EXC(regs)	(((regs)->trap & 8) != 0)
 #endif /* __powerpc64__ */
+#define TRAP(regs)		((regs)->trap & ~TRAP_FLAGS_MASK)
 
 static __always_inline void set_trap(struct pt_regs *regs, unsigned long val)
 {
@@ -222,12 +227,12 @@ static inline bool trap_is_syscall(struct pt_regs *regs)
 
 static inline bool trap_norestart(struct pt_regs *regs)
 {
-	return regs->trap & 0x10;
+	return regs->trap & 0x1;
 }
 
 static __always_inline void set_trap_norestart(struct pt_regs *regs)
 {
-	regs->trap |= 0x10;
+	regs->trap |= 0x1;
 }
 
 #define arch_has_single_step()	(1)