System calls 拦截syscall，但不是sysret，为什么？

我想拦截syscall/sysret， 所以我取消了EFER.SCE， 所以syscall/sysret将被困到Xen中，然后我模拟syscall/sysret。 但是我只看到syscall，没有看到一个sysret，客户机照常运行。 谁能给我一个提示

来宾VM是Xun-4.1.4上运行的xubuntu_14_x64。 代码如下，我只做了一点修改，有什么问题吗

取消设置EFER.SCE的代码如下

                case HVM_MIT_SYSCALL_ring :
                {
                    struct vcpu   *v;
                    for_each_vcpu (d, v)
                    {
                        v->arch.hvm_vcpu.guest_efer  =  v->arch.hvm_vcpu.guest_efer  &  ~EFER_SCE;
                    }
                    break;
                }

        case TRAP_invalid_op:
    {
        vmx_vmexit_ud_intercept(regs);
        break;

    }

要处理的代码如下所示

                case HVM_MIT_SYSCALL_ring :
                {
                    struct vcpu   *v;
                    for_each_vcpu (d, v)
                    {
                        v->arch.hvm_vcpu.guest_efer  =  v->arch.hvm_vcpu.guest_efer  &  ~EFER_SCE;
                    }
                    break;
                }

        case TRAP_invalid_op:
    {
        vmx_vmexit_ud_intercept(regs);
        break;

    }

模拟syscall和sysret的代码如下所示，这是Xen的源代码。我只注释 生成异常ifmsr\u内容&EFER\u SCE==0，EXC\u UD，-1

    case 0x05: /* syscall */ {
    uint64_t msr_content;
    struct segment_register cs = { 0 }, ss = { 0 };
    int rc;
    printk("emulate syscall");

    generate_exception_if(in_realmode(ctxt, ops), EXC_UD, -1);
    generate_exception_if(!in_protmode(ctxt, ops), EXC_UD, -1);

    /* Inject #UD if syscall/sysret are disabled. */
    fail_if(ops->read_msr == NULL);
    if ( (rc = ops->read_msr(MSR_EFER, &msr_content, ctxt)) != 0 )
        goto done;
    //generate_exception_if((msr_content & EFER_SCE) == 0, EXC_UD, -1);

    if ( (rc = ops->read_msr(MSR_STAR, &msr_content, ctxt)) != 0 )
        goto done;

    msr_content >>= 32;
    cs.sel = (uint16_t)(msr_content & 0xfffc);
    ss.sel = (uint16_t)(msr_content + 8);

    cs.base = ss.base = 0; /* flat segment */
    cs.limit = ss.limit = ~0u;  /* 4GB limit */
    cs.attr.bytes = 0xc9b; /* G+DB+P+S+Code */
    ss.attr.bytes = 0xc93; /* G+DB+P+S+Data */

  #ifdef __x86_64__
    rc = in_longmode(ctxt, ops);
    printk(" in longmode  \n");
    if ( rc < 0 )
        goto cannot_emulate;
    if ( rc )
    {
        cs.attr.fields.db = 0;
        cs.attr.fields.l = 1;

        _regs.rcx = _regs.rip;
        _regs.r11 = _regs.eflags & ~EFLG_RF;

        if ( (rc = ops->read_msr(mode_64bit() ? MSR_LSTAR : MSR_CSTAR,
                                 &msr_content, ctxt)) != 0 )
            goto done;
        _regs.rip = msr_content;

        if ( (rc = ops->read_msr(MSR_FMASK, &msr_content, ctxt)) != 0 )
            goto done;
        _regs.eflags &= ~(msr_content | EFLG_RF);
    }
    else
   #endif
    {
        printk(" not here\n  ");
        if ( (rc = ops->read_msr(MSR_STAR, &msr_content, ctxt)) != 0 )
            goto done;

        _regs.ecx = _regs.eip;
        _regs.eip = (uint32_t)msr_content;
        _regs.eflags &= ~(EFLG_VM | EFLG_IF | EFLG_RF);
    }

    fail_if(ops->write_segment == NULL);
    if ( (rc = ops->write_segment(x86_seg_cs, &cs, ctxt)) ||
         (rc = ops->write_segment(x86_seg_ss, &ss, ctxt)) )
        goto done;

    break;
}

case 0x06: /* clts */
    generate_exception_if(!mode_ring0(), EXC_GP, 0);
    fail_if((ops->read_cr == NULL) || (ops->write_cr == NULL));
    if ( (rc = ops->read_cr(0, &dst.val, ctxt)) ||
         (rc = ops->write_cr(0, dst.val&~8, ctxt)) )
        goto done;
    break;

case 0x07: 
    printk("emulate sysret, \n");
    break;