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 <dvlas...@redhat.com> >> CC: Paolo Bonzini <pbonz...@redhat.com> >> CC: Avi Kivity <avi.kiv...@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? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
