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

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Date:   Wed, 10 Feb 2021 12:17:37 +0100
From:   Alexander Graf <graf@...zon.com>
To:     Hikaru Nishida <hikalium@...omium.org>,
        <linux-kernel@...r.kernel.org>
CC:     <suleiman@...gle.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>,
        KVM list <kvm@...r.kernel.org>, <mtosatti@...hat.com>
Subject: Re: [RFC PATCH 0/2] Introduce a way to adjust CLOCK_BOOTTIME from
 userspace for VM guests



On 10.02.21 11:39, Hikaru Nishida wrote:
> 
> From: Hikaru Nishida <hikalium@...omium.org>
> 
> 
> Hi folks,
> 
> 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 don't fully grasp why you want the guest to manually adjust its 
CLOCK_BOOTTIME. Wouldn't it make more sense to extend kvmclock's notion 
of wall clock time to tell you about suspended vs executed wall clock?


Alex




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