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Subject: Re: [RFC PATCH 0/2] Introduce a way to adjust CLOCK_BOOTTIME from
 userspace for VM guests
To:     Hikaru Nishida <hikalium@chromium.org>,
        <linux-kernel@vger.kernel.org>
CC:     <suleiman@google.com>, Andra Paraschiv <andraprs@amazon.com>,
        "Arnd Bergmann" <arnd@arndb.de>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        "Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        John Stultz <john.stultz@linaro.org>,
        Kurt Kanzenbach <kurt@linutronix.de>,
        Linus Walleij <linus.walleij@linaro.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Stephen Boyd <sboyd@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        KVM list <kvm@vger.kernel.org>, <mtosatti@redhat.com>
References: <20210210103908.1720658-1-hikalium@google.com>
From:   Alexander Graf <graf@amazon.com>
Message-ID: <d9127c33-e067-9b49-5985-0b09e3ede279@amazon.com>
Date:   Wed, 10 Feb 2021 12:17:37 +0100
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On 10.02.21 11:39, Hikaru Nishida wrote:
> =

> From: Hikaru Nishida <hikalium@chromium.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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