x86, mm: fault.c, refactor/simplify the is_prefetch() code

Impact: no functionality changed

Factor out the opcode checker into a helper inline.

The code got a tiny bit smaller:

   text	   data	    bss	    dec	    hex	filename
   4632	     32	     24	   4688	   1250	fault.o.before
   4618	     32	     24	   4674	   1242	fault.o.after

And it got cleaner / easier to review as well.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/mm/fault.c

@@ -99,38 +99,12 @@ static inline int notify_page_fault(struct pt_regs *regs)
  *
  * Opcode checker based on code by Richard Brunner.
  */
-static int
-is_prefetch(struct pt_regs *regs, unsigned long error_code, unsigned long addr)
+static inline int
+check_prefetch_opcode(struct pt_regs *regs, unsigned char *instr,
+		      unsigned char opcode, int *prefetch)
 {
-	unsigned char *max_instr;
-	unsigned char *instr;
-	int scan_more = 1;
-	int prefetch = 0;
-
-	/*
-	 * If it was a exec (instruction fetch) fault on NX page, then
-	 * do not ignore the fault:
-	 */
-	if (error_code & PF_INSTR)
-		return 0;
-
-	instr = (unsigned char *)convert_ip_to_linear(current, regs);
-	max_instr = instr + 15;
-
-	if (user_mode(regs) && instr >= (unsigned char *)TASK_SIZE)
-		return 0;
-
-	while (scan_more && instr < max_instr) {
-		unsigned char instr_hi;
-		unsigned char instr_lo;
-		unsigned char opcode;
-
-		if (probe_kernel_address(instr, opcode))
-			break;
-
-		instr_hi = opcode & 0xf0;
-		instr_lo = opcode & 0x0f;
-		instr++;
+	unsigned char instr_hi = opcode & 0xf0;
+	unsigned char instr_lo = opcode & 0x0f;
 
 	switch (instr_hi) {
 	case 0x20:
@@ -141,8 +115,7 @@ is_prefetch(struct pt_regs *regs, unsigned long error_code, unsigned long addr)
 		 * opcode if some of these prefixes are present so
 		 * X86_64 will never get here anyway
 		 */
-			scan_more = ((instr_lo & 7) == 0x6);
-			break;
+		return ((instr_lo & 7) == 0x6);
 #ifdef CONFIG_X86_64
 	case 0x40:
 		/*
@@ -152,30 +125,57 @@ is_prefetch(struct pt_regs *regs, unsigned long error_code, unsigned long addr)
 		 * but for now it's good enough to assume that long
 		 * mode only uses well known segments or kernel.
 		 */
-			scan_more = (!user_mode(regs)) || (regs->cs == __USER_CS);
-			break;
+		return (!user_mode(regs)) || (regs->cs == __USER_CS);
 #endif
 	case 0x60:
 		/* 0x64 thru 0x67 are valid prefixes in all modes. */
-			scan_more = (instr_lo & 0xC) == 0x4;
-			break;
+		return (instr_lo & 0xC) == 0x4;
 	case 0xF0:
 		/* 0xF0, 0xF2, 0xF3 are valid prefixes in all modes. */
-			scan_more = !instr_lo || (instr_lo>>1) == 1;
-			break;
+		return !instr_lo || (instr_lo>>1) == 1;
 	case 0x00:
 		/* Prefetch instruction is 0x0F0D or 0x0F18 */
-			scan_more = 0;
-
 		if (probe_kernel_address(instr, opcode))
-				break;
-			prefetch = (instr_lo == 0xF) &&
+			return 0;
+
+		*prefetch = (instr_lo == 0xF) &&
 			(opcode == 0x0D || opcode == 0x18);
-			break;
+		return 0;
 	default:
-			scan_more = 0;
-			break;
+		return 0;
 	}
+}
+
+static int
+is_prefetch(struct pt_regs *regs, unsigned long error_code, unsigned long addr)
+{
+	unsigned char *max_instr;
+	unsigned char *instr;
+	int prefetch = 0;
+
+	/*
+	 * If it was a exec (instruction fetch) fault on NX page, then
+	 * do not ignore the fault:
+	 */
+	if (error_code & PF_INSTR)
+		return 0;
+
+	instr = (void *)convert_ip_to_linear(current, regs);
+	max_instr = instr + 15;
+
+	if (user_mode(regs) && instr >= (unsigned char *)TASK_SIZE)
+		return 0;
+
+	while (instr < max_instr) {
+		unsigned char opcode;
+
+		if (probe_kernel_address(instr, opcode))
+			break;
+
+		instr++;
+
+		if (!check_prefetch_opcode(regs, instr, opcode, &prefetch))
+			break;
 	}
 	return prefetch;
 }