Re: [libvirt] Exporting kvm_max_guest_tsc_khz to userspace (was Re: [PATCH 4/4] kvm: Allow migration with invtsc)

On Mon, Jan 09, 2017 at 03:58:11PM +0100, Paolo Bonzini wrote: > > > On 08/01/2017 21:28, Eduardo Habkost wrote: >>> Well just export KVM_GET_TSC_KHZ in a QMP command right? Its pretty >>> easy. >>> >>> Let me know if you need any help coding or testing. >> I just found out that KVM doesn't provide something that QEMU and >> libvirt need: the value of kvm_max_guest_tsc_khz. Without it, we >> have no way to know if a given VM is really migratable to a host. >> >> Could we add a KVM_CAP_MAX_TSC_KHZ capability for that? > > The ratio is really quite high, 256x the host frequency for AMD and > 65536x for Intel. Anything below 2^32 Hz (above that, there would > probably be other failures) is safe. 2^32 Hz (~4.3 GHz) sounds like a limit likely to be hit by future CPUs. Which kind of failures do you think we could see?