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List:       kvm
Subject:    [PATCH 2/2 v2] KVM: x86 emulator: Avoid pushing back ModRM byte fetched for group decoding
From:       Takuya Yoshikawa <takuya.yoshikawa () gmail ! com>
Date:       2012-04-30 8:48:25
Message-ID: 20120430174825.6716c422a20f1f25e5cd3adc () gmail ! com
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From: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>

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

Now that ModRM flag can be found in the top level opcode tables, fetch
ModRM byte before group decoding to make the code simpler.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
---
 arch/x86/kvm/emulate.c |    8 +++-----
 1 files changed, 3 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 8d2c3d0..7fd2576 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);
@@ -3976,17 +3975,16 @@ done_prefixes:
 	}
 	ctxt->d = opcode.flags;
 
+	if (ctxt->d & ModRM)
+		ctxt->modrm = insn_fetch(u8, ctxt);
+
 	while (ctxt->d & GroupMask) {
 		switch (ctxt->d & GroupMask) {
 		case Group:
-			ctxt->modrm = insn_fetch(u8, ctxt);
-			--ctxt->_eip;
 			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];
-- 
1.7.5.4

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