Re: [Qemu-devel] [PATCH] Fix serial interface vmstate

[Pavel Dovgaluk]

> On 2011-06-22 11:15, Pavel Dovgaluk wrote:
> >>>>>   What is the purpose of subsections?
> >>>>
> >>>> To skip the new fields whenever possible. That would allow to continue
> >>>> saving a vmstate on a new version of qemu and then restoring it on an
> >>>> older one.
> >>>
> >>>   Do you have an idea how to implement "needed" function for my case?
> >>> Because I think, these fields should always be saved and loaded, because
> >>> they are related to the main state of the interface, not the kind of
> >>> optional substate.
> >>
> >> E.g., if the fifo is empty, you do not need to save its content. That
> >> would be one part of the condition. Go through all fields and check if
> >> they have states that could be ignored or if they could be ignored if
> >> other already saved fields have specific values. If you find any new
> >> field that must always be restored, let us discuss it. It may turn out
> >> that a substate is unrealistic, then we need to go with a new version.
> >
> >   You mean, if FIFO is empty an will not be saved, we will have to clear
> > it before loading every time?
> 
> We (soon, patches posted) do a reset before every loadvm. We already do
> this when starting a new machine. That sets all states that aren't
> restored to defaults. In case of the fifo, they are all emptied.
> 
> >   So there should be multiple subsections for every possible field?
> >   E.g. timers are saved only if they are pending, thr_ipending is saved
> > only when it is nonzero, and so on. Do you mean that?
> 
> Nope, only a single subsection. Either we are able to avoid any new
> field under certain circumstances or we have to break the legacy format
> anyway.

  Fields added by my patch are not belong to single subsection.
  Consider THR and TSR registers:
  THR:
      Bit 5 in the LSR, line status register can be used to check if new 
      information must be written to THR. The value 1 indicates that the 
      register is empty.
  TSR:
      Contains data, that should be shifted out. This register is
      Interconnected with transmit_timer - when timer event occurs,
      the data is shifted out.
  There are also other fields, that describe the state of the serial
interface, but cannot be directly derived from THR and TSR state.

  So, there are the following ways, as I see it:
  1. Make several subsections - one for every substate.
  2. Make one subsection with very complex "needed" condition,
     which will be true, when any part of the subsection should be
     written. I think, that such complex condition will be too hard
     for testing.
  3. Change version of the format.
  

Pavel Dovgaluk