linux-kernel - Re: [PATCH v4 00/10] make L2's kvm-clock stable, get rid of pvclock_gtod

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Message-ID: <CALAqxLVngARqG4g8URMQGQtZec_p5NVP7hFXQGS6+JWp74FVMA@mail.gmail.com>
Date:   Wed, 2 Aug 2017 09:49:48 -0700
From:   John Stultz <john.stultz@...aro.org>
To:     Denis Plotnikov <dplotnikov@...tuozzo.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        lkml <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>, rkagan@...tuozzo.com,
        den@...tuozzo.com
Subject: Re: [PATCH v4 00/10] make L2's kvm-clock stable, get rid of
 pvclock_gtod_copy in KVM

On Wed, Aug 2, 2017 at 7:38 AM, Denis Plotnikov
<dplotnikov@...tuozzo.com> wrote:
> V4:
>   * removed "is stable" function with vague definition of stability
>     there is the only function which does time with cycle stamp getting
>   * some variables renamed
>   * some patches split into smaller once
>   * atomic64_t usage is replaced with atomic_t
>
> V3:
>   Changing the timekeeper interface for clocksource reading looks like
>   an overkill to achive the goal of getting cycles stamp for KVM.
>   Instead extend the timekeeping interface and add functions which provide
>   necessary data: read clocksource with cycles stamp, check whether the
>   clock source is stable.
>
>   Use those functions and improve existing timekeeper functionality to
>   replace pvclock_gtod_copy scheme in masterclock data calculation.
>
> V2:
>   The main goal is to make L2 kvm-clock be stable when it's running over L1
>   with stable kvm-clock.
>
>   The patch series is for x86 architecture only. If the series is approved
>   I'll do changes for other architectures but I don't have an ability to
>   compile and check for every single on (help needed)
>
>   The patch series do the following:
>
>         * change timekeeper interface to get cycles stamp value from
>           the timekeeper
>         * get rid of pvclock copy in KVM by using the changed timekeeper
>           interface: get time and cycles right from the timekeeper
>         * make KVM recognize a stable kvm-clock as stable clocksource
>           and use the KVM masterclock in this case, which means making
>           L2 stable when running over stable L1 kvm-clock

So, from a brief skim, I'm not a big fan of this patchset. Though this
is likely in part due to that I haven't seen anything about *why*
these changes are needed.

Can you briefly explain the issue you're trying to solve, and why you
think this approach is the way to go?
(Its usually a good idea to have such rational included in the patchset)

thanks
-john