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

[Denys Vlasenko <dvlasenk@redhat.com>]

On 07/04/2013 11:13 AM, Gleb Natapov wrote:
> 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.

Can you elaborate?

0x90 is special-cased in CPU to be a NOP regardless of bit width.
IOW, xchg %eax,%eax ordinarily would clear upper 32 bits of %rax,
but 0x90 doesn't do that.

Do you mean that with REX.R==1, 0x90 will refer to R8?