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daa2369 "spapr_pci: Add a 64-bit MMIO window" subtly broke migration from qemu-2.7 to the current version. It split the device's MMIO window into two pieces for 32-bit and 64-bit MMIO. The patch included backwards compatibility code to convert the old property into the new format. However, the property value was also transferred in the migration stream and compared with a (probably unwise) VMSTATE_EQUAL. So, the "raw" value from 2.7 is compared to the new style converted value from (pre-)2.8 giving a mismatch and migration failure. Along with the actual field that caused the breakage, there are several other ill-advised VMSTATE_EQUAL()s. To fix forwards migration, we read the values in the stream into scratch variables and ignore them, instead of comparing for equality. To fix backwards migration, we populate those scratch variables in pre_save() with adjusted values to match the old behaviour. To permit the eventual possibility of removing this cruft from the stream, we only include these compatibility fields if a new 'pre-2.8-migration' property is set. We clear it on the pseries-2.8 machine type, which obviously can't be migrated backwards, but set it on earlier machine type versions. Signed-off-by: David Gibson <david@gibson.dropbear.id.au> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com> Reviewed-by: Thomas Huth <thuth@redhat.com> Reviewed-by: Greg Kurz <groug@kaod.org> Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>