On Wed, 13 Nov 2013 00:10:39 +0200 "Michael S. Tsirkin" <m...@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
