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

[Gleb Natapov]

On Sun, Dec 20, 2009 at 08:43:38AM -0600, Anthony Liguori wrote:
> 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).
> 
That much I noticed :), but first after reboot new ROMs should take effect.
Does this happed? Second what if ROM size have changed (on destination it is
smaller)? And third what about roms that are loaded with fw_cfg interface (me
mentioning vgabios was not accident :))

--
                        Gleb.