[Xen-devel] Re: [PATCH][RFC] Emulating real mode with x86_emulate

[Anthony Liguori <aliguori@xxxxxxxxxx>]

Kamble, Nitin A wrote:
> Anthony,
> eip of 0x0 is the right address.
>
> Because the trampoline_Start code is relocated to address 0.
>
>     /* Relocate real-mode trampoline to 0x0. */
>     "    mov  $trampoline_start,%esi \n"
>     "    xor  %edi,%edi              \n"
>     "    mov  $trampoline_end,%ecx   \n"
>     "    sub  %esi,%ecx              \n"
>     "    rep  movsb                  \n"
>
> Still there is no progress in the emulation. The guest is dying with 
> triple-fault just like before.
> Thanks & Regards,
> Nitin
> Open Source Technology Center, Intel Corporation.
> -------------------------------------------------------------------------
> The mind is like a parachute; it works much better when it's open.
>
>
> On Fri, 2007-03-30 at 14:08 -0700, Nitin A Kamble wrote:
> > Hi Anthony,
> >
> > On Fri, 2007-03-30 at 14:00 -0500, Anthony Liguori wrote:
> > >     /* Enter real mode, reload all segment registers and IDT. */
> > >     "    ljmp $0x8,$0x0              \n"
> > >     "trampoline_start: .code16       \n"
> > >     "    mov  %eax,%cr0              \n"
> > >
> > > That change was pretty recent so unless it was reverted it should Just Work.
> > >
> > > Regards,
> > >
> > > Anthony Liguori
> > >       
> > Now I see cr0 is updated after returning from main.
> >    I enabled the debug log in the xen code now.
> >
> > (XEN) HVM2: Creating MP tables 
> > ...                                             
> > (XEN) HVM2: Loading Cirrus VGABIOS 
> > ...                                         
> > (XEN) HVM2: Loading ACPI 
> > ...                                                   
> > (XEN) HVM2: Loading VMXAssist ... 
> > deadbeef                                     
> > (XEN) HVM2: 
> > foo                                                                
> > (XEN) 
> > hvmop_emulate_realmode                                                    
> >
> > (XEN) guest requests real mode 
> > emulation                                       
> > (XEN) foo 
> > 221                                                                  
> > (XEN) HVM2: Invoking ROMBIOS 
> > ...                                               
> > (XEN) vmx_vmexit_handler called. eip = 
> > 0x0                                     
> > (XEN) vmx_cr_access called 
> > eip=0x0                                             
> > (XEN) mov_to_cr 0 called 
> > eip=0x0                                               
> > (XEN) vmx_set_cr0 called 
> > eip=0x0                                               
> > (XEN) Transfering -- control to x86_emulate eip 
> > 0x0                            
> > (XEN) hvm.c:446:d2 Triple fault on VCPU0 - invoking HVM system reset. 
> >
> > It shows cr0 is getting modified. But the eip is still 0x0.
set_cr0 is returning 1 though which should increment eip to the next 
instruction.
I'm a bit perplexed about my eip now and also why your eip is still 0.  
It should be the instruction following the mov cr0.
Regards,

Anthony Liguori

> > Thanks & Regards,
> > Nitin
> > Open Source Technology Center, Intel Corporation.
> > -------------------------------------------------------------------------
> > The mind is like a parachute; it works much better when it's open.

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