[Qemu-devel] Debugging guest OS

[Qemu-devel] Debugging guest OS

[Mariano Wahlmann]

I want to know, if it's posible to debug an guest OS like bochs, i mean 
disassemble current instruction, step by step running, possibility of 
change registers values, etc. I try to use GDB, but i think this is for 
debbuging qemu itself.
any clues?

thanks

[Qemu-devel] Re: Debugging guest OS

[Ben Pfaff]

Mariano Wahlmann <wahlmann@agro.uba.ar> writes:

> I want to know, if it's posible to debug an guest OS like bochs, i
> mean disassemble current instruction, step by step running,
> possibility of change registers values, etc. I try to use GDB, but i
> think this is for debbuging qemu itself.

You can use gdb on the guest if you invoke qemu with -s.
-- 
Ben Pfaff 
email: blp@cs.stanford.edu
web: http://benpfaff.org

Re: [Qemu-devel] Re: Debugging guest OS

[Mariano Wahlmann]

Ben Pfaff wrote:

>Mariano Wahlmann <wahlmann@agro.uba.ar> writes:
>
>  
>
>>I want to know, if it's posible to debug an guest OS like bochs, i
>>mean disassemble current instruction, step by step running,
>>possibility of change registers values, etc. I try to use GDB, but i
>>think this is for debbuging qemu itself.
>>    
>>
>
>You can use gdb on the guest if you invoke qemu with -s.
>  
>
that's right, thanks
but in order to disassemble i have to do "display /i $cs * 16 + $eip", 
this is for real mode,  what is the syntaxis for protected mode?
is there any varible that contains a pointer to current virtual address 
of the next instruction.


-- 
________________________________________________________
Mariano Agustín Wahlmann
Administrador de Red
Facultad de Agronomía - Buenos Aires - Argentina
Te.: (+54 11) 4524-8000 int.8108
email: mailto:wahlmann@agro.uba.ar
www: http://www.agro.uba.ar

Re: [Qemu-devel] Re: Debugging guest OS

[Paul Brook]

On Thursday 02 June 2005 16:20, Mariano Wahlmann wrote:
> but in order to disassemble i have to do "display /i $cs * 16 + $eip",
> this is for real mode,  what is the syntaxis for protected mode?
> is there any varible that contains a pointer to current virtual address
> of the next instruction.

Usually it will just be $eip

Technically you need to add the current code segment offset, but most (all?) 
x86 OS use a flat memory model, so this is zero.

Paul

Re: [Qemu-devel] Re: Debugging guest OS

[Mariano Wahlmann]

Paul Brook wrote:

>On Thursday 02 June 2005 16:20, Mariano Wahlmann wrote:
>  
>
>>but in order to disassemble i have to do "display /i $cs * 16 + $eip",
>>this is for real mode,  what is the syntaxis for protected mode?
>>is there any varible that contains a pointer to current virtual address
>>of the next instruction.
>>    
>>
>
>Usually it will just be $eip
>
>Technically you need to add the current code segment offset, but most (all?) 
>x86 OS use a flat memory model, so this is zero.
>
>Paul
>
>  
>
i think this is not true, you can have so many desciptors on gdt table, 
not all start at has 0  as base address.
The way that i find to do that, is to look in "info register" on qemu 
monitor, and look what is the current base for cs and add it to eip.
This is a very uneasy way.
 

>_______________________________________________
>Qemu-devel mailing list
>Qemu-devel@nongnu.org
>http://lists.nongnu.org/mailman/listinfo/qemu-devel
>
>  
>

-- 
________________________________________________________
Mariano Agustín Wahlmann
Administrador de Red
Facultad de Agronomía - Buenos Aires - Argentina
Te.: (+54 11) 4524-8000 int.8108
email: mailto:wahlmann@agro.uba.ar
www: http://www.agro.uba.ar

Re: [Qemu-devel] Re: Debugging guest OS

[Paul Brook]

On Thursday 02 June 2005 17:32, Mariano Wahlmann wrote:
> Paul Brook wrote:
> >On Thursday 02 June 2005 16:20, Mariano Wahlmann wrote:
> >>but in order to disassemble i have to do "display /i $cs * 16 + $eip",
> >>this is for real mode,  what is the syntaxis for protected mode?
> >>is there any varible that contains a pointer to current virtual address
> >>of the next instruction.
> >
> >Usually it will just be $eip
> >
> >Technically you need to add the current code segment offset, but most
> > (all?) x86 OS use a flat memory model, so this is zero.
> >
> >Paul
>
> i think this is not true, you can have so many desciptors on gdt table,
> not all start at has 0  as base address.
> The way that i find to do that, is to look in "info register" on qemu
> monitor, and look what is the current base for cs and add it to eip.
> This is a very uneasy way.

Right, that's why I said "usually" and "most".

Paul

Re: [Qemu-devel] Re: Debugging guest OS

[Fabrice Bellard]

Mariano Wahlmann wrote:
> 
> 
> Ben Pfaff wrote:
> 
>> Mariano Wahlmann <wahlmann@agro.uba.ar> writes:
>>
>>  
>>
>>> I want to know, if it's posible to debug an guest OS like bochs, i
>>> mean disassemble current instruction, step by step running,
>>> possibility of change registers values, etc. I try to use GDB, but i
>>> think this is for debbuging qemu itself.
>>>   
>>
>>
>> You can use gdb on the guest if you invoke qemu with -s.
>>  
>>
> that's right, thanks
> but in order to disassemble i have to do "display /i $cs * 16 + $eip", 
> this is for real mode,  what is the syntaxis for protected mode?
> is there any varible that contains a pointer to current virtual address 
> of the next instruction.

In QEMU you have $pc which gives the correct value.

Fabrice.