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

[Gleb Natapov <gleb@redhat.com>]

On Thu, Jul 04, 2013 at 10:58:29AM +0200, Denys Vlasenko wrote:
> 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@redhat.com>
> CC: Paolo Bonzini <pbonzini@redhat.com>
> CC: Avi Kivity <avi.kivity@gmail.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;
This does not work on 64bit and REX prefix.

> +	case 0x91 ... 0x97: /* xchg reg, rax */
>  		rc = em_xchg(ctxt);
>  		break;
>  	case 0x98: /* cbw/cwde/cdqe */
> -- 
> 1.8.1.4

--
			Gleb.