[PATCH 4/4] KVM: x86 emulator: Avoid pushing back ModRM byte in decode_opcode()

Takuya Yoshikawa
2012-04-23T15:37:17Z

From: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>

Although ModRM byte is read for group decoding, it is soon pushed back
to make decode_modrm() fetch it later.

We should consistently read it, only once, in decode_opcode() instead.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Cc: Takuya Yoshikawa <takuya.yoshikawa@gmail.com>
---
arch/x86/kvm/emulate.c | 19 ++++++++++++-------
1 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index e87570e..8729773 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -972,7 +972,6 @@ static int decode_modrm(struct x86_emulate_ctxt *ctxt,
ctxt->modrm_rm = base_reg = (ctxt->rex_prefix & 1) << 3; /* REG.B */
}

- ctxt->modrm = insn_fetch(u8, ctxt);
ctxt->modrm_mod |= (ctxt->modrm & 0xc0) >> 6;
ctxt->modrm_reg |= (ctxt->modrm & 0x38) >> 3;
ctxt->modrm_rm |= (ctxt->modrm & 0x07);
@@ -3969,6 +3968,7 @@ done:
* @ctxt: emulation context
*
* Decodes opcode bytes and reads opcode from table.
+ * The ModRM byte, if exists, is also fetched into ctxt->modrm.
*
* Returns X86EMUL_CONTINUE on success.
*/
@@ -3977,6 +3977,8 @@ static int decode_opcode(struct x86_emulate_ctxt *ctxt)
int rc = X86EMUL_CONTINUE;
int goffset, simd_prefix;
struct opcode opcode;
+ bool modrm_fetched = false;
+ u64 gtype;

/* Two-byte opcode? */
if (ctxt->b == 0x0f) {
@@ -3988,17 +3990,18 @@ static int decode_opcode(struct x86_emulate_ctxt *ctxt)

ctxt->d = opcode.flags;

- while (ctxt->d & GroupMask) {
- switch (ctxt->d & GroupMask) {
- case Group:
+ while ((gtype = ctxt->d & GroupMask)) {
+ if (!modrm_fetched && gtype != Prefix) {
ctxt->modrm = insn_fetch(u8, ctxt);
- --ctxt->_eip;
+ modrm_fetched = true;
+ }
+
+ switch (gtype) {
+ case Group:
goffset = (ctxt->modrm >> 3) & 7;
opcode = opcode.u.group[goffset];
break;
case GroupDual:
- ctxt->modrm = insn_fetch(u8, ctxt);
- --ctxt->_eip;
goffset = (ctxt->modrm >> 3) & 7;
if ((ctxt->modrm >> 6) == 3)
opcode = opcode.u.gdual->mod3[goffset];
@@ -4039,6 +4042,8 @@ static int decode_opcode(struct x86_emulate_ctxt *ctxt)
ctxt->check_perm = opcode.check_perm;
ctxt->intercept = opcode.intercept;

+ if (!modrm_fetched && (ctxt->d & ModRM))
+ ctxt->modrm = insn_fetch(u8, ctxt);
done:
return rc;
}
--
1.7.5.4