Re: [libvirt] TSC frequency configuration & invtsc migration (was Re: [PATCH 4/4] kvm: Allow migration with invtsc)

On Wed, Jan 04, 2017 at 11:39:16AM -0200, Eduardo Habkost wrote: > On Wed, Jan 04, 2017 at 09:56:56AM -0200, Marcelo Tosatti wrote: > > On Tue, Dec 27, 2016 at 05:21:20PM -0200, Eduardo Habkost wrote: > > > Instead of blocking migration on the source when invtsc is > > > enabled, rely on the migration destination to ensure there's no > > > TSC frequency mismatch. > > > > > > We can't allow migration unconditionally because we don't know if > > > the destination is a QEMU version that is really going to ensure > > > there's no TSC frequency mismatch. To ensure we are migrating to > > > a destination that won't ignore SET_TSC_KHZ errors, allow invtsc > > > migration only on pc-*-2.9 and newer. > > > > > > Signed-off-by: Eduardo Habkost <ehabkost(a)redhat.com&gt; [...] > > > @@ -2655,12 +2656,14 @@ int kvm_arch_put_registers(CPUState *cpu, int level) > > > } > > > > > > if (level == KVM_PUT_FULL_STATE) { > > > - /* We don't check for kvm_arch_set_tsc_khz() errors here, > > > - * because TSC frequency mismatch shouldn't abort migration, > > > - * unless the user explicitly asked for a more strict TSC > > > - * setting (e.g. using an explicit "tsc-freq" option). > > > + /* Migration TSC frequency mismatch is fatal only if we are > > > + * actually reporting Invariant TSC to the guest. > > > */ > > > - kvm_arch_set_tsc_khz(cpu); > > > + ret = kvm_arch_set_tsc_khz(cpu); > > > + if ((x86_cpu->env.features[FEAT_8000_0007_EDX] & CPUID_APM_INVTSC) && > > > + ret < 0) { > > > + return ret; > > > + } > > > } > > > > Will the guest continue in the source in this case? > > > > I think this is past the point where migration has been declared > > successful. > > > > Otherwise looks good. > > Good point. I will make additional tests and see if there's some > other place where the kvm_arch_set_tsc_khz() call can be moved > to. So, if we solve this and do something on (for example) post_load, we still have a problem: device state is migrated after RAM. This means QEMU will check for TSC scaling and abort migration very late. We could solve that by manually registering a SaveVMHandler that will send the TSC frequency on save_live_setup, so migration is aborted earlier. But: this sounds like just a complex hack to work around the real problems: 1) TSC frequency is guest-visible, and anything that affects guest ABI should depend on the VM configuration, not on host capabilities;

2) Setting TSC frequency depending on the host will make migratability unpredictable for management software: the same VM config could be migratable to host A when started on host B, but not migratable to host A when started on host C.

I suggest we allow migration with invtsc if and only if tsc-frequency is set explicitly by management software. In other words, apply only patches 1/4 and 2/4 from this series. After that, we will need libvirt support for configuring tsc-frequency.