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

Paolo Bonzini
2017-01-11T14:06:09Z

On 11/01/2017 14:26, Eduardo Habkost wrote:
> 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?

Just integer overflows. Current CPUs have been at 3.9 GHz for a while now.

Paolo