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. > > 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. > 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? -- Thanks, David / dhildenb
