Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20210210103908.1720658-1-hikalium@google.com>
In-Reply-To: <20210210103908.1720658-1-hikalium@google.com>
From:   Arnd Bergmann <arnd@kernel.org>
Date:   Wed, 10 Feb 2021 14:32:19 +0100
Message-ID: <CAK8P3a1ViL91_uEtOwuvTi34A9gMPF-+82eOLNKh8knt+h811A@mail.gmail.com>
Subject: Re: [RFC PATCH 0/2] Introduce a way to adjust CLOCK_BOOTTIME from
 userspace for VM guests
To:     Hikaru Nishida <hikalium@chromium.org>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        suleiman@google.com, Alexander Graf <graf@amazon.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>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Wed, Feb 10, 2021 at 11:39 AM Hikaru Nishida <hikalium@chromium.org> wrote:
> From: Hikaru Nishida <hikalium@chromium.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