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

[Eduardo Habkost <ehabkost@redhat.com>]

On Fri, Jan 06, 2017 at 08:31:27AM -0200, Marcelo Tosatti wrote:
[...]
> > > 
> > > > Maybe your use case just needs a explicit "invtsc-migration=on"
> > > > command-line flag without a mechanism to abort migration on
> > > > mismatch? I can't tell.
> > > 
> > > Again, there is no special case.
> > > 
> > > > Note that even if we follow your suggestion and implement an
> > > > alternative version of patch 4/4 to cover your use case, I will
> > > > strongly recommend libvirt developers to support configuring TSC
> > > > frequency if they want to support invtsc + migration without
> > > > surprising/unpredictable restrictions on migratability.
> > > 
> > > Well, alright. If you make the TSC frequency of the host
> > > available to mgmt software as you describe, and write the steps mgmt
> > > software should take, i'm good.
> > 
> > I plan to. The problem is that the mechanism to query the host
> > frequency may be unavailable in the first version.
> 
> 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?

-- 
Eduardo