Re: [Xen-devel] there is no sysret in X86_emulate, why?

[hanyandong <hanyandong@xxxxxxxxx>]

> -----原始邮件-----
> 发件人: "Jan Beulich" <JBeulich@xxxxxxxx>
> 发送时间: 2014年10月30日 星期四
> 收件人: "Christoph Egger" <chegger@xxxxxxxxx>, hanyandong <hanyandong@xxxxxxxxx>
> 抄送: xen-devel@xxxxxxxxxxxxx
> 主题: Re: [Xen-devel] there is no sysret in X86_emulate, why?
> 
> >>> On 30.10.14 at 02:46, <hanyandong@xxxxxxxxx> wrote:
> > (1)In x86_emulate(), there are sysenter/sysexit, syscall. But why no sysret?
> 
> Perhaps on the basis that this already when introduced was only
> meant to be usable on 64-bit hypervisors, and 64-bit capable CPUs
> always support SYSRET (whereas the scope of support for the
> other three varies)? Christoph, you added that code years ago - is
> there any other explanation for this?
> 
> > (2)I want to iuntercept syscall/sysret, so I unset the EFER.SCE, so 
> > syscall/sysret will trap into Xen, then I emulate syscall/sysret.
> > But  I only see syscall and did not see one sysret,  the guest run as usual. 
> >  any one can give me an hint?
> 
> Assuming you did everything correctly, this seems odd. But in any
> event I'd suggest confirming such behavior in a native environment
> first.
> 
> Jan
>

thanks for your reply.

the guest VM is xubuntu_14_x64 running on Xen-4.1.4.

the code is as below , I only did a little modification ， anything is wrong? 

the code to unset EFER.SCE is as below

                    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;
                    }

the code to handle #UP is as below

        case TRAP_invalid_op:
        {
            vmx_vmexit_ud_intercept(regs);
            break;

}

the code to emulate syscall and sysret is as below, this is the source code of Xen. I only annotate the " generate_exception_if((msr_content & EFER_SCE) == 0, EXC_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;

> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxx
> http://lists.xen.org/xen-devel



--
Best Regards,
yandong

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel