Re: [Qemu-devel] [PATCH v4 2/2] loader: store FW CFG ROM files in RAM

[Paolo Bonzini <pbonzini@redhat.com>]

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 <mst@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