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Re: [Qemu-devel] Exporting kvm_max_guest_tsc_khz to userspace (was Re: [

From: Eduardo Habkost
Subject: Re: [Qemu-devel] Exporting kvm_max_guest_tsc_khz to userspace (was Re: [PATCH 4/4] kvm: Allow migration with invtsc)
Date: Wed, 11 Jan 2017 11:26:53 -0200
User-agent: Mutt/1.7.1 (2016-10-04)

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?


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