linux-kernel - Re: [PATCH v2] x86/kvm/emulate.c: simplify NOP (opcode 0x90) check

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Message-ID: <20130704113554.GK5113@redhat.com>
Date:	Thu, 4 Jul 2013 14:35:54 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
	Avi Kivity <avi.kivity@...il.com>
Subject: Re: [PATCH v2] x86/kvm/emulate.c: simplify NOP (opcode 0x90) check

Please send kvm patches to kvm@...r.kernel.org too.

On Thu, Jul 04, 2013 at 01:27:00PM +0200, Denys Vlasenko wrote:
> We do not need to check for reg == RAX for opcodes 0x91...0x97.
> 
The is no reason to optimize here. This is not performance sensitive
path, far from it. If not for "clear high bits" behaviour of em_xchg() we
would just call it to emulate nop.

> Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
> CC: Paolo Bonzini <pbonzini@...hat.com>
> CC: Avi Kivity <avi.kivity@...il.com>
> 
> 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 | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 2bc1e81..2dc48a1 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -4544,9 +4544,14 @@ special_insn:
>  	case 0x8d: /* lea r16/r32, m */
>  		ctxt->dst.val = ctxt->src.addr.mem.ea;
>  		break;
> -	case 0x90 ... 0x97: /* nop / xchg reg, rax */
> +	case 0x90: /* nop / xchg reg, rax */
> +		/* Not all opcodes 90 are NOPs, only ones which refer to RAX */
> +		/* For example, 49 90 is xchg %rax,%r8 */
> +		/* Bare 90 and 40 90 (redundant REX prefix) are nop */
>  		if (ctxt->dst.addr.reg == reg_rmw(ctxt, VCPU_REGS_RAX))
>  			break;
> +		/* Fall through */
> +	case 0x91 ... 0x97: /* xchg reg, rax */
>  		rc = em_xchg(ctxt);
>  		break;
>  	case 0x98: /* cbw/cwde/cdqe */
> -- 
> 1.8.1.4

--
			Gleb.
--
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