linux-kernel - Re: [patch 0/3] KVM CPU frequency change hypercalls

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Date:   Thu, 23 Feb 2017 20:19:26 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Radim Krcmar <rkrcmar@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [patch 0/3] KVM CPU frequency change hypercalls

On Thu, Feb 23, 2017 at 06:35:24PM +0100, Paolo Bonzini wrote:
> 
> 
> 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.

This is how cpufreq-userspace works:

2.2 Governor
------------

On all other cpufreq implementations, these boundaries still need to
be set. Then, a "governor" must be selected. Such a "governor" decides
what speed the processor shall run within the boundaries. One such
"governor" is the "userspace" governor. This one allows the user - or
a yet-to-implement userspace program - to decide what specific speed
the processor shall run at.

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

Whats the current usecase, or forseeable future usecase, for save/restore
across preemption again? (which would validate the broken by design
claim).

> 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.
> 
> Paolo

Hypercalls overcome the problems mentioned in the first email
of the thread, i think you missed them:

"[patch 0/3] KVM CPU frequency change hypercalls"