From: Paolo Bonzini <pbonzini@redhat.com>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Gleb Natapov <gleb@redhat.com>,
linux-kernel@vger.kernel.org, Avi Kivity <avi.kivity@gmail.com>
Subject: Re: [PATCH] x86/kvm/emulate.c: simplify NOP (opcode 0x90) check
Date: Thu, 04 Jul 2013 13:08:08 +0200 [thread overview]
Message-ID: <51D55798.5090404@redhat.com> (raw)
In-Reply-To: <51D55728.4060909@redhat.com>
Il 04/07/2013 13:06, Denys Vlasenko ha scritto:
>>> >> - 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?
Yes.
$ echo 'xchg %rax,%r8' | as
$ objdump -d a.out
a.out: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <.text>:
0: 49 90 xchg %rax,%r8
Paolo
prev parent reply other threads:[~2013-07-04 11:08 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-07-04 8:58 [PATCH] x86/kvm/emulate.c: simplify NOP (opcode 0x90) check Denys Vlasenko
2013-07-04 9:13 ` Gleb Natapov
2013-07-04 11:06 ` Denys Vlasenko
2013-07-04 11:07 ` Gleb Natapov
2013-07-04 11:08 ` Paolo Bonzini [this message]
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