Il 19/08/2013 16:26, Michael S. Tsirkin ha scritto: > ROM files that are put in FW CFG are copied to guest ram, by BIOS, but > they are not backed by RAM so they don't get migrated. > > Each time we change two bytes in such a ROM this breaks cross-version > migration: since we can migrate after BIOS has read the first byte but > before it has read the second one, getting an inconsistent state. > > Future-proof this by creating, for each such ROM, > an MR serving as the backing store. > This MR is never mapped into guest memory, but it's registered > as RAM so it's migrated with the guest. > > Naturally, this only helps for -M 1.7 and up, older machine types > will still have the cross-version migration bug. > Luckily the race window for the problem to trigger is very small, > which is also likely why we didn't notice the cross-version > migration bug in testing yet. > > Signed-off-by: Michael S. Tsirkin <m...@redhat.com>
This doesn't really help much unless we support migration of different-sized RAM regions, does it? In particular, what happens if the region is smaller on the destination? Do we get garbage written out of a malloc-ed region's bounds? The patches look good, but I'm not sure they are an improvement yet. Paolo
