Re: [Qemu-devel] qemu log function to print out the registers of the guest

[Steven]

On Thu, Aug 16, 2012 at 12:54 PM, Peter Maydell
<address@hidden> wrote:
> On 16 August 2012 17:36, Steven <address@hidden> wrote:
>> I would like to get a trace of guest memory access. So I can not use
>> "info registers".
>> What I want to do is that when tcg fetches a load instruction at
>> disas_insns(), the guest memory address should be calculated.
>
> You cannot calculate the guest memory address at the point where
> TCG is translating the load instruction. This is because that
> address depends on the values of guest registers at runtime.
> At translation time these values are not known. Also they may
> be different for different runs through the same generated code.

Thanks. Then what I thought is wrong.

>
> QEMU is a just-in-time translator (JIT). For a JIT it is
> important to remember the difference between:
>  * translation time. Here we know what the guest code (instructions)
> are, but we do not know what the guest CPU registers will be
>  * run time. This may be some time later, and we may execute
> the same code several times. We don't have any access to
> information about the guest code we are running unless
> we specifically recorded it at translation time.

Take this in_asm as example, mov 0x4(%ebx)  %eax. I saw the translated
host code for this single load instruction (using -d in_asm,out_asm)
are

OUT: [size=107]
0x4025d890:  mov    0x28(%r14),%rbp
0x4025d894:  add    $0xc,%rbp
0x4025d898:  mov    %ebp,%ebp
...
0x4025d8e8:  mov    %rbp,0x8(%r14)
0x4025d8ec:  xor    %eax,%eax
0x4025d8ee:  mov    $0x7fc1a598d176,%r10
0x4025d8f8:  jmpq   *%r10
So the run time function should be tcg_out_qemu_ld, right?
Could you provide some suggestion where I should add the record
information to help translate the guest memory address? Thanks.


>
> When you are reading (or trying to change) QEMU source code
> you need to know whether the QEMU code will be running at
> translation or run time. The answer affects what information
> you have access to, and what you can do to the guest.
>
> -- PMM