Messages in this thread | | | Date | Thu, 4 Jul 2013 12:13:46 +0300 | From | Gleb Natapov <> | Subject | Re: [PATCH] x86/kvm/emulate.c: simplify NOP (opcode 0x90) check |
| |
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.
| |