linux-kernel - Re: [RFC PATCH 0/2] Introduce a way to adjust CLOCK

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Date:   Wed, 10 Feb 2021 14:32:19 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Hikaru Nishida <hikalium@...omium.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        suleiman@...gle.com, Alexander Graf <graf@...zon.com>,
        Andra Paraschiv <andraprs@...zon.com>,
        Arnd Bergmann <arnd@...db.de>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        John Stultz <john.stultz@...aro.org>,
        Kurt Kanzenbach <kurt@...utronix.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 0/2] Introduce a way to adjust CLOCK_BOOTTIME from
 userspace for VM guests

On Wed, Feb 10, 2021 at 11:39 AM Hikaru Nishida <hikalium@...omium.org> wrote:
> From: Hikaru Nishida <hikalium@...omium.org>
>
> We'd like to add a sysfs interface that enable us to advance
> CLOCK_BOOTTIME from userspace. The use case of this change is that
> adjusting guest's CLOCK_BOOTTIME as host suspends to ensure that the
> guest can notice the device has been suspended.
> We have an application that rely on the difference between
> CLOCK_BOOTTIME and CLOCK_MONOTONIC to detect whether the device went
> suspend or not. However, the logic did not work well on VM environment
> since most VMs are pausing the VM guests instead of actually suspending
> them on the host's suspension.
> With following patches, we can adjust CLOCK_BOOTTIME without actually
> suspending guest and make the app working as intended.
> I think this feature is also useful for other VM solutions since there
> was no way to do this from userspace.
>
> As far as I checked, it is working as expected but is there any concern
> about this change? If so, please let me know.

I think the correct internal interface to call would be
timekeeping_inject_sleeptime64(), which changes boottime in a
safe way.

Not sure what should call it, but kvmclock as Alex suggested might
be the right place.

         Arnd