"Dr. David Alan Gilbert" <dgilb...@redhat.com> wrote: > * Jaggi, Manish (manish.ja...@cavium.com) wrote: > >> Just to add what happens in ARM64 case, qemu running on Machine A sends cpu >> state information to Machine B. >> This state contains MIDR value, and so Processor ID value is compared in KVM >> and not in qemu (correcting myself). >> >> IIRC, Peter/Eric please point if there is something incorrect in the below >> flow... >> >> (Machine B) >> target/arm/machine.c: cpu_post_load() >> - updates cpu->cpreg_values[i] : which includes MIDR (processor >> ID register) >> >> - calls write_list_to_kvmstate(cpu, KVM_PUT_FULL_STATE) >> >> target/arm/kvm.c: write_list_to_kvmstate >> - calls => kvm_vcpu_ioctl(cs, KVM_SET_ONE_REG, >> &r); >> >> => and it eventually lands up IIRC in >> Linux code in >> >> => >> arch/arm64/kvm/sys_regs.c : set_invariant_sys_reg(u64 id, void __user *uaddr) >> /* This is what >> we mean by invariant: you can't change it. */ >> if (r->val != >> val) >> return >> -EINVAL; >> Note: MIDR_EL1 >> is invariant register. >> result: Migration fails on Machine B. >> >> A few points: >> - qemu on arm64 is invoked with -machine virt and -cpu as host. So we don't >> explicitly define which cpu. > > Note that even on x86 we don't guarantee as much about '-cpu host', what > we expect to work for migration is that if you pick a '-cpu amodel' > and both hosts support the feature flags required by 'amodel' then it > should work.
I really think that the right approach here is not using -cpu host. You do the full work, create a model as David says, and be sure that you car run that model on both cpus. It is a lot of work, but it is the only way to make sure that this is going to work long term. >> - In case Machine A and Machine B have almost same Core and the >> delta may-not have any effect on qemu operation, migration should >> work by just looking into whitelist. >> whitelist can be given as a parameter for qemu on machine B. >> >> qemu-system-aarch64 -whitelist <ids separated by commas> >> >> (This is my proposal) >> >> - So in cpu_post_load (Machine B) qemu can lookup whitelist and >> replace the MIDR with the one at Machine B. >> Sounds good? >> >> - Juan raised a point about clock speed, I am not sure it will have >> any effect on arm since qemu is run with -cpu host param. >> I could be wrong here, Peter/Eric can you please correct me... > > Clock speed is only really a problem for things like timestamp counters; > some cores let you scale them; for those that don't then yes it's a bit > odd. This was just one example that I thought on top of my head. Other thing that I remember was that at some point, if you migrated to a cpu with hyperthreading, you lost half of your performance counters. I still think that you need to create a proper model, and work from there. Later, Juan.
