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Date:   Thu, 23 Feb 2017 18:35:24 +0100
From:   Paolo Bonzini <>
To:     Radim Krcmar <>,
        Marcelo Tosatti <>
        "Rafael J. Wysocki" <>,
        Viresh Kumar <>
Subject: Re: [patch 0/3] KVM CPU frequency change hypercalls

On 03/02/2017 20:09, Radim Krcmar wrote:
> One reason why we have a kernel/userspace split is to allow sharing of
> CPU time.  Each application then its state that the kernel keeps track
> of and saves/restores while time-multiplexing.
> Our frequency scaling interface goes against the idea -- guest kernel
> cannot schedule multiple userspaces on the same vCPU, because they could
> conflict by overriding frequency.
> i.e. our feature implies userspace tasks pinned to isolated vCPUs.

That's bad.  This feature is broken by design unless it does proper
save/restore across preemption.

You don't need a hypercall.  Add a cpufreq driver in DPDK that doesn't
use sysfs, and connect it to a daemon in the host through virtio-serial
or vsock.


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