linux-kernel - [PATCH] x86/kvm/emulate.c: simplify NOP (opcode 0x90) check

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Date:	Thu,  4 Jul 2013 10:58:29 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>, linux-kernel@...r.kernel.org
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Avi Kivity <avi.kivity@...il.com>
Subject: [PATCH] x86/kvm/emulate.c: simplify NOP (opcode 0x90) check

The check sits in switch() statement which itself can check
for opcode 0x90 far more efficiently.

On assembler level, this change simply eliminates the following
bit of code:

4c 8b a3 d8 00 00 00    mov    0xd8(%rbx),%r12
31 f6                   xor    %esi,%esi
48 89 df                mov    %rbx,%rdi
e8 32 8c ff ff          callq  490 <reg_read>
48 8d 83 78 01 00 00    lea    0x178(%rbx),%rax
83 8b 70 01 00 00 01    orl    $0x1,0x170(%rbx)
83 8b 74 01 00 00 01    orl    $0x1,0x174(%rbx)
49 39 c4                cmp    %rax,%r12
0f 84 88 fa ff ff       je     7304 <x86_emulate_insn+0x204>

Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
CC: Paolo Bonzini <pbonzini@...hat.com>
CC: Avi Kivity <avi.kivity@...il.com>
---
 arch/x86/kvm/emulate.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 2bc1e81..7e6e74a 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -4544,9 +4544,9 @@ special_insn:
 	case 0x8d: /* lea r16/r32, m */
 		ctxt->dst.val = ctxt->src.addr.mem.ea;
 		break;
-	case 0x90 ... 0x97: /* nop / xchg reg, rax */
-		if (ctxt->dst.addr.reg == reg_rmw(ctxt, VCPU_REGS_RAX))
-			break;
+	case 0x90: /* nop */
+		break;
+	case 0x91 ... 0x97: /* xchg reg, rax */
 		rc = em_xchg(ctxt);
 		break;
 	case 0x98: /* cbw/cwde/cdqe */
-- 
1.8.1.4

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