Re: [Qemu-devel] invtsc + migration + TSC scaling

[Paolo Bonzini]

On 17/10/2016 16:50, Radim Krčmář wrote:
> 2016-10-17 07:47-0200, Marcelo Tosatti:
>> On Fri, Oct 14, 2016 at 06:20:31PM -0300, Eduardo Habkost wrote:
>>> I have been wondering: should we allow live migration with the
>>> invtsc flag enabled, if TSC scaling is available on the
>>> destination?
>>
>> TSC scaling and invtsc flag, yes.
> 
> Yes, if we have well synchronized time between hosts, then we might be
> able to migrate with a TSC shift that cannot be perceived by the guest.
> 
> Unless the VM also has a migratable assigned PCI device that uses ART,
> because we have no protocol to update the setting of ART (in CPUID), so
> we should keep migration forbidden then.

We don't publish the ART leaf at all, do we?

>> 1) Migration: to host with different TSC frequency.
> 
> We shouldn't have done this even now when emulating anything newer than
> Pentium 4, because those CPUs have constant TSC, which only lacks the
> guarantee that it doesn't stop in deep C-states:

Right, but:

>> since Linux guests use kvmclock and Windows guests use Hyper-V
>> enlightenment, it should be fine to disable 2).

... and 1 too.

We should also blacklist the TSC deadline timer when invtsc is not
available.

Paolo