* Liran Alon (liran.alon@xxxxxxxxxx) wrote: > Hi, > > Looking at QEMU source code, I am puzzled regarding how migration backwards compatibility is preserved regarding X86CPU. > > As I understand it, fields that are based on KVM capabilities and guest runtime usage are defined in VMState subsections in order to not send them if not necessary. > This is done such that in case they are not needed and we migrate to an old QEMU which don’t support loading this state, migration will still succeed > (As .needed() method will return false and therefore this state won’t be sent as part of migration stream). > Furthermore, in case .needed() returns true and old QEMU don’t support loading this state, migration fails. As it should because we are aware that guest state > is not going to be restored properly on destination. > > I’m puzzled about what will happen in the following scenario: > 1) Source is running new QEMU with new KVM that supports save of some VMState subsection. > 2) Destination is running new QEMU that supports load this state but with old kernel that doesn’t know how to load this state. > > I would have expected in this case that if source .needed() returns true, then migration will fail because of lack of support in destination kernel. > However, it seems from current QEMU code that this will actually succeed in many cases. > > For example, if msr_smi_count is sent as part of migration stream (See vmstate_msr_smi_count) and destination have has_msr_smi_count==false, > then destination will succeed loading migration stream but kvm_put_msrs() will actually ignore env->msr_smi_count and will successfully load guest state. > Therefore, migration will succeed even though it should have failed… > > It seems to me that QEMU should have for every such VMState subsection, a .post_load() method that verifies that relevant capability is supported by kernel > and otherwise fail migration. > > What do you think? Should I really create a patch to modify all these CPUX86 VMState subsections to behave like this? I don't know the x86 specific side that much; but from my migration side the answer should mostly be through machine types - indeed for smi-count there's a property 'x-migrate-smi-count' which is off for machine types pre 2.11 (see hw/i386/pc.c pc_compat_2_11) - so if you've got an old kernel you should stick to the old machine types. There's nothing guarding running the new machine type on old-kernels; and arguably we should have a check at startup that complains if your kernel is missing something the machine type uses. However, that would mean that people running with -M pc would fail on old kernels. A post-load is also a valid check; but one question is whether, for a particular register, the pain is worth it - it depends on the symptom that the missing state causes. If it's minor then you might conclude it's not worth a failed migration; if it's a hung or corrupt guest then yes it is. Certainly a warning printed is worth it. Dave > Thanks, > -Liran -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
