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>

1 files changed, 6 insertions, 0 deletions

diff --git a/arch/x86/lib/insn.c b/arch/x86/lib/insn.c
index 1313ae6b478b..5ea00ddc1825 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;
                 }