Re: [Qemu-devel] [PATCH RFC 0/5] New VMState table based load/save infrastructure

[Gerd Hoffmann <kraxel@redhat.com>]

> 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.

Yes, we want.  And I think the right approach to do that is to simply 
use the old, existing code and switch based on version_id.  To stick 
with the apic example:  When loadvm finds a v2 (or older) apic section, 
go call apic_load.  When loadvm finds a v3 (or newer) apic section, call 
the new generic load code with the apic vmstatefield 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.

Yes, we want.  PCI devices call a generic function to save pci state. 
We want a common pci vmstatefield list too and have some way to refer to 
them from the device tables.

> 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?

No.  We want fixed-sized sections, bonus points for a 'size' field in 
the header.  Just save everything unconditionally.  The current 
save/load functions do way too much stuff conditionally.  Saving a few 
bytes simply isn't worth the complexity of getting that right.

> 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?

Handle compatibility by keeping the old load functions and start over 
with a clean sheet.

cheers,
   Gerd