Re: [Qemu-devel] [PATCH] linux-user: fix wait* syscall status returns

[Alexander Graf]

On 24.11.2011, at 00:02, Peter Maydell wrote:

> On 23 November 2011 23:00, Alexander Graf <address@hidden> wrote:
>> 
>> On 23.11.2011, at 23:34, Peter Maydell wrote:
>> 
>>> On 23 November 2011 22:03, Alexander Graf <address@hidden> wrote:
>>>> Yes. Maybe we should add a check if input_state != output_state and
>>>> only then do the conversion?
>>> 
>>> I'm not sure this works (unless you go to the effort of implementing
>>> target_to_host_waitstatus() which seems overkill to me) but I'm not
>>> entirely sure what you're proposing to compare to what.
>> 
>> It's an integer. If the number is the same before and after the wait
>> syscall, we can safely assume that it's the same thing, so we don't
>> have to convert it, no?
> 
> But you don't know whether it's the same before and after because
> wait() didn't write to it [=> don't write to guest memory] or if
> wait() did write to it but happened to write it as the same value
> it had before [=> do write to guest memory].

If it was the same value before, it will still be the same value in guest 
memory.

  get_guest_s32(status, status_ptr);
  old_status = status;
  wait(...)
  if (old_status != status) {
    status = convert_status(status);
    put_guest_s32(status, status_ptr);
  }

If the values are identical, it's safe to assume that we don't have to convert. 
And the value will already be in guest memory :)


Alex