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

[Steven <wangwangkang@gmail.com>]

On Thu, Aug 16, 2012 at 12:54 PM, Peter Maydell
<peter.maydell@linaro.org> wrote:
> On 16 August 2012 17:36, Steven <wangwangkang@gmail.com> 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.
>
> 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.

To verify what is translation time and what is the run time, I log the
register information before disassembling each guest code. I copied
some results from the log file, which is generated at run time of a
guest machine.

         EAX=00000000  EBX=00006ffc
         IN:
         0x00000000000f2087:  mov    $0xf5588,%eax

         EAX=000f5588  EBX=00006ffc
         IN:
         0x00000000000f208B:  move 0x4(%ebx)  %eax

The first instruction load eax with the value 0xf5588, so the eax at
the second instruction is EAX=000f5588. So can I consider the memory
address of 0x4(%ebx) as (00006ffc +  4)? I think this should be the
run time information I need. Please correct me  if there is anything
wrong. 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