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

[Takuya Yoshikawa <takuya.yoshikawa@xxxxxxxxx>]

From: Takuya Yoshikawa <yoshikawa.takuya@xxxxxxxxxxxxx>

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@xxxxxxxxxxxxx>
Cc: Takuya Yoshikawa <takuya.yoshikawa@xxxxxxxxx>
---
 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

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