[Qemu-devel] [PATCH 0/3] New VMState table based load/save infrastructure

[Juan Quintela <quintela@redhat.com>]

His this is 2nd version of the series, I changed:

v2:
* Add _V() constructors.  We almost never need the version field.
* Add const to VMStateDescription uses (BlueWirl suggestion)
* Add const to VMStateInfo uses
* Remove VMStateInfo parameter from get/put callbacks. Not needed.
* Load of old versions is done with old foo_load function (Kraxel suggestion)
* Add struct support
* Move i8254 to new infrastructure, to test struct support
* I removed the autostart patches, updated version sent to list

i8254 note:
There is only one timer in the 1st channel, in the other two channels,
the timer is not created ever, this is the reason why I sent the irq_timer
not in the irq channels.

ToDo:
- still not optional test function.  Notice that this can be done
  with a local VMStateInfo struct.
- Start/end functions, not done waiting for user case (next goal is virtio
  and I think we need them there).
- For structures, Kraxel suggested to use a VMSTATE_INCLUDE() instead of
  having to declare a new VMStateInfo struct.  As I already have the new
  struct code working, I sent it with the struct.  Thinking about how to
  implement the VMSTATE_INCLUDE() and which one is easier to use.

Comments?

v1:

This series implement a new save/load approach for device state.  I want to
start a discussion on how is better to design it.  This is the reason why only
one device has been ported to this new approach.

I increased the version_id of apic state, just to sent the 3 arrays
(isr, tmr, irr) as arrays.  I didn't want to introduce structure support
before there is any discussion about this, not that it is imposible to do
it in a compatible way.

Problems of current approach:
- have to maintain both load and save functions on sync
- it is not type safe
- you can't use easily the device save/load information to do more things.
  For instance, print the state of a device in the monitor. To do this, you
  basically have to copy foo_apic() function.
- You can't examine the saved images in any meaningful way

New approach:
- you only have to declare the state once, in a table, save/load functions are
  generic for all devices.
- it is type safe
- it is qdev-like design (i.e. you don't have to learn yet a completely
  different appearch for a similar problem)
- it is trivial to add new operations to VMStateInfo, and then you can use
  them anywhere (for instance printing in the monitor, etc).

Problems of new approach:
- it is less flexible, you can't progamatically save in any order that you want.
  It is not clear to me that we _want_ that flexibility.  Look for instance
  and hw/msix.c and think if you can be sure that save/load are on sync, and
  as soon as we get new versions, that things could be tested to be wright.

What do I want to discuss:
a- do we want to be able to load state form old versions?
   I am not sure that we are going to get this right for complex devices,
   and I don't completely see how we can have any kind of testing here.
   There was already a discussion about this on the list.
b- Is this the right approach?  What more do we want/need?
   For instance, implementing struct save support, and calling
   other "sub-descriptions" is not difficult, we just have to decide
   if we want it.
c- In the current approach, we have loops to send arrays,  I think that one
   got already done better on new approarch.  But we don't have support
   for ifs (see hw/ide.c
       if (s->identify_set) {
        qemu_get_buffer(f, (uint8_t *)s->identify_data, 512);
    }
   Do we want support for things like that?
d- how aggresive should the new design be?  i.e. be able to be compatible with
   old design is good, or can we start with a clean sheet and just remove the
   gotchas of the previous design?



Juan Quintela (3):
  New VMstate save/load infrastructure
  Port apic to new VMState design
  Port i8254 to new VMState design

 hw/apic.c  |   67 ++++++-------
 hw/hw.h    |  153 ++++++++++++++++++++++++++++
 hw/i8254.c |   72 ++++++++------
 savevm.c   |  325 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 4 files changed, 548 insertions(+), 69 deletions(-)