Re: [Qemu-devel] [PATCH 2/2] pc: add 'etc/reserved-memory-end' fw_cfg interface for SeaBIOS

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 13 Nov 2013 00:10:39 +0200
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Tue, Nov 12, 2013 at 07:26:02PM +0100, Paolo Bonzini wrote:
> > Il 12/11/2013 14:58, Igor Mammedov ha scritto:
> > > 'etc/reserved-memory-end' will allow QEMU to tell BIOS where PCI
> > > BARs mapping could safely start in high memory.
> > > 
> > > Allowing BIOS to start mapping 64-bit PCI BARs at address where it
> > > wouldn't conflict with other mappings QEMU might place before it.
> > > 
> > > That permits QEMU to reserve extra address space before
> > > 64-bit PCI hole for memory hotplug.
> > 
> > I may be royally wrong, but I think the new file should only be added to
> > new machine types.  Otherwise, after migrating old machine types from
> > new QEMU to old QEMU, you may end up with PCI BARs mapped outside the
> > "PCI windows" that exist until before patch 1/2 of this series.
> > 
> > Does this make sense?
> 
> Yes.
> Generally FW CFG must not be added/removed for a given machine types,
> otherwise guest that is migrated while reading it will
> get a corrupted result: half old and half new.

Is it true for a file 'etc/reserved-memory-end' though?

I've debugged SeaBIOS to learn more about it, and new->old migration with
following reboot, showed that file is not found by SeaBIOS (well since old
QEMU doesn't have it), as result SeaBIOS fallbacks to the old behavior 
placing 64-PCI bars right above ram_over_4G as it was intended.

And with 'etc/reserved-memory-end' == ram_over_4G_end as it is in this
patch, there isn't issue whatsoever.

Looks like there is no migrations issues with files, provided SeaBIOS knows
how to deal with a missing file.

-- 
Regards,
  Igor