powerpc/lib/sstep: Add cmpb instruction emulation

This patch adds emulation of the cmpb instruction, enabling xmon to
emulate this instruction.
Tested for correctness against the cmpb asm instruction on ppc64le.
Signed-off-by: Matt Brown <matthew.brown.dev@gmail.com>
Reviewed-by: Cyril Bur <cyrilbur@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/lib/sstep.c

@@ -596,6 +596,22 @@ static nokprobe_inline void do_cmp_unsigned(struct pt_regs *regs, unsigned long
 	regs->ccr = (regs->ccr & ~(0xf << shift)) | (crval << shift);
 }
 
+static nokprobe_inline void do_cmpb(struct pt_regs *regs, unsigned long v1,
+				unsigned long v2, int rd)
+{
+	unsigned long long out_val, mask;
+	int i;
+
+	out_val = 0;
+	for (i = 0; i < 8; i++) {
+		mask = 0xffUL << (i * 8);
+		if ((v1 & mask) == (v2 & mask))
+			out_val |= mask;
+	}
+
+	regs->gpr[rd] = out_val;
+}
+
 static nokprobe_inline int trap_compare(long v1, long v2)
 {
 	int ret = 0;
@@ -1064,6 +1080,10 @@ int analyse_instr(struct instruction_op *op, struct pt_regs *regs,
 			do_cmp_unsigned(regs, val, val2, rd >> 2);
 			goto instr_done;
 
+		case 508: /* cmpb */
+			do_cmpb(regs, regs->gpr[rd], regs->gpr[rb], ra);
+			goto instr_done;
+
 /*
  * Arithmetic instructions
  */