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

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Date:   Fri, 24 Feb 2017 13:17:07 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Marcelo Tosatti <mtosatti@...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 24/02/2017 12:50, Marcelo Tosatti wrote:
>>>
>>> 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.
>> The userspace program sets a policy for the whole system.
> No, its per cpu.

Yeah, what I mean is that userspace program can be per-CPU, but it looks
at all the processes running on that CPU ("the whole system").  This is
very different from a guest, which is isolated.

>>>> 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).
>> Stop a guest that is using cpufreq, start a guest that is not using it.
>> The second guest's performance now depends on the state that the first
>> guest left in cpufreq.
>
> Nothing forbids the host to implement switching with the
> current hypercall interface: all you need is a scheduler
> hook.

Can it be done in vcpu_load/vcpu_put?  But you still would have two
components (KVM and sysfs) potentially fighting over the frequency, and
that's still a bit ugly.

Paolo