* David Hildenbrand ([email protected]) wrote: > On 14.02.20 11:25, Dr. David Alan Gilbert wrote: > > * David Hildenbrand ([email protected]) wrote: > >> Resizing while migrating is dangerous and does not work as expected. > >> The whole migration code works on the usable_length of ram blocks and does > >> not expect this to change at random points in time. > >> > >> Precopy: The ram block size must not change on the source, after > >> ram_save_setup(), so as long as the guest is still running on the source. > >> > >> Postcopy: The ram block size must not change on the target, after > >> synchronizing the RAM block list (ram_load_precopy()). > >> > >> AFAIKS, resizing can be trigger *after* (but not during) a reset in > >> ACPI code by the guest > >> - hw/arm/virt-acpi-build.c:acpi_ram_update() > >> - hw/i386/acpi-build.c:acpi_ram_update() > >> > >> I see no easy way to work around this. Fail hard instead of failing > >> somewhere in migration code due to strange other reasons. AFAIKs, the > >> rebuilts will be triggered during reboot, so this should not affect > >> running guests, but only guests that reboot at a very bad time and > >> actually require size changes. > >> > >> Let's further limit the impact by checking if an actual resize of the > >> RAM (in number of pages) is required. > >> > >> Don't perform the checks in qemu_ram_resize(), as that's called during > >> migration when syncing the used_length. Update documentation. > > > > Interesting; we need to do something about this - but banning resets > > during migration is a bit harsh; and aborting the source VM is really > > nasty - for a precopy especially we shouldn't kill the source VM, > > we should just abort the migration. > > Any alternative, easy solutions to handle this? I do wonder how often > this will actually trigger in real life.
Well it's not that hard to abort a migration (I'm not sure we've got a convenient wrapper to do it - but it shouldn't be hard to add). > > > > The other thing that worries me is that acpi_build_update calls > > acpi_ram_update->memory_region_ram_resize > > multiple times. > > It's different memory regions, no? table_mr, rsdp_mr, linker_mr. Oh, so it is. > > So, it might be that the size you end up with at the end of > > acpi_build_update is actually the same size as the original - so > > the net effect is the RAMBlock didn't really get resized. > > Are you sure? No! Avocado has a migration+reset test, so it's worth trying. Certainly in a cloud setup migrations happen often and no one knows what the guest is doing; aborting the source isn't acceptable. It surprises me a bit that the region sizes would change due to guest actions - I thought they were determined by the set of virtual hardware; not sure if a hot-unplug/plug followed by reset would trigger it or not. Dave > > -- > Thanks, > > David / dhildenb -- Dr. David Alan Gilbert / [email protected] / Manchester, UK
