x86/asm/decoder: Create artificial 3rd byte for 2-byte VEX

Before this patch, users need to do this to fetch vex.vvvv:

if (insn->vex_prefix.nbytes == 2) {
vex_vvvv = ((insn->vex_prefix.bytes[1] >> 3) & 0xf) ^ 0xf;
}
if (insn->vex_prefix.nbytes == 3) {
vex_vvvv = ((insn->vex_prefix.bytes[2] >> 3) & 0xf) ^ 0xf;
}

Make it so that insn->vex_prefix.bytes[2] always contains
vex.wvvvvLpp bits.

Signed-off-by: Denys Vlasenko <[e-mail]>
Acked-by: Masami Hiramatsu <[e-mail]>
Cc: Ananth N Mavinakayanahalli <[e-mail]>
Cc: Frank Ch. Eigler <[e-mail]>
Cc: Jim Keniston <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Srikar Dronamraju <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/lib/insn.c

···
 				/* VEX.W overrides opnd_size */
 				insn->opnd_bytes = 8;
 		} else {
+			/*
+			 * For VEX2, fake VEX3-like byte#2.
+			 * Makes it easier to decode vex.W, vex.vvvv,
+			 * vex.L and vex.pp. Masking with 0x7f sets vex.W == 0.
+			 */
+			insn->vex_prefix.bytes[2] = b2 & 0x7f;
 			insn->vex_prefix.nbytes = 2;
 			insn->next_byte += 2;
 		}