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

[tip-bot for Denys Vlasenko <tipbot@zytor.com>]

Commit-ID:  8a764a875fe3cf3a83296bacd00bfc41917e95e2
Gitweb:     http://git.kernel.org/tip/8a764a875fe3cf3a83296bacd00bfc41917e95e2
Author:     Denys Vlasenko <dvlasenk@redhat.com>
AuthorDate: Thu, 12 Feb 2015 20:04:39 +0100
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 18 Feb 2015 21:01:50 +0100

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 <dvlasenk@redhat.com>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Cc: Frank Ch. Eigler <fche@redhat.com>
Cc: Jim Keniston <jkenisto@linux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/1423767879-31691-1-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/lib/insn.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/lib/insn.c b/arch/x86/lib/insn.c
index 1313ae6..5ea00dd 100644
--- a/arch/x86/lib/insn.c
+++ b/arch/x86/lib/insn.c
@@ -164,6 +164,12 @@ found:
 				/* 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;
 		}