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

From: Paolo Bonzini
Subject: Re: [Qemu-devel] Exporting kvm_max_guest_tsc_khz to userspace (was Re: [PATCH 4/4] kvm: Allow migration with invtsc)
Date: Mon, 9 Jan 2017 15:58:11 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1

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.


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