Messages in this thread | | | Date | Thu, 4 Jul 2013 14:07:47 +0300 | From | Gleb Natapov <> | Subject | Re: [PATCH] x86/kvm/emulate.c: simplify NOP (opcode 0x90) check |
| |
On Thu, Jul 04, 2013 at 01:06:16PM +0200, Denys Vlasenko wrote: > 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? Yes.
-- Gleb.
| |