[Qemu-devel] Re: [SeaBIOS] [PATCH 0/8] option rom loading overhaul.

[Anthony Liguori]

Gleb Natapov wrote:
> On Fri, Dec 18, 2009 at 12:01:06PM +0100, Gerd Hoffmann wrote:
>   
> > From: Anthony Liguori <address@hidden>
> >
> >   Hi,
> >
> > Option rom saga continued.  This is the first series I consider ready
> > for merging.  Changes:
> >
> >   * pci roms: will be loaded via option pci rom bar.
> >   * non-pci roms: will be loaded via fw_cfg.
> >
> > Note that using the old bochs-bios based pc-bios will stop working
> > with this patch series applied.
> >
> > The last two patches of this series are *not* intended to be merged,
> > they are just for testing convinience.  They carry a new seabios
> > binary and the changes needed and turn on bios debug messages in qemu.
> >
> > Seabios patches will be posted shortly as separate patch series.
> >
> >     
> I see that this is merged already, but I have a question. How migration
> during ROM loading suppose to work?

Magically :-)

Really, we have a fundamentally problem with live migration (that's 
orthogonal to this series).  We allocate memory via qemu_ram_alloc() but 
we don't have any context to that allocation.  When we migrate memory, 
we do so by transferring ram basically in allocation order.

If for some reason allocation order changes, badness ensues.  The 
problem is, a lot of devices use qemu_ram_alloc() now and they do so 
when they are initialized so the stability of the allocation depends on 
the initialization order.  It's exacerbated by things like PCI hotplug.

We need to make a change in the live migration protocol that transfers 
memory identified with tuples (id, chunk offset).  We'll need to change 
every qemu_ram_alloc() to take an id argument too.

>  What will happed if we migrate to newer
> qemu with updated vgabios ROM while BIOS is in the middle of loading it on the
> source? It seems that destination will have garbled ROM loaded.
>   

During device init, pci device allocates memory, loads rom into 
allocated memory.  Upon live migration, allocated memory gets over 
written by rom contents from source.  So in this case, it will actually 
work today (assuming you get stable ram allocation offsets).

Regards,

Anthony Liguori