linux-kernel - Re: [RFC PATCH 2/2] drivers/virt: introduce CLOCK_BOOTTIME adjustment sysfs interface driver

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Message-ID: <87eehoax14.fsf@nanos.tec.linutronix.de>
Date:   Wed, 10 Feb 2021 14:10:31 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Hikaru Nishida <hikalium@...omium.org>,
        linux-kernel@...r.kernel.org
Cc:     suleiman@...gle.com, Hikaru Nishida <hikalium@...omium.org>,
        Alexander Graf <graf@...zon.com>,
        Andra Paraschiv <andraprs@...zon.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Masahiro Yamada <masahiroy@...nel.org>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH 2/2] drivers/virt: introduce CLOCK_BOOTTIME adjustment sysfs interface driver

On Wed, Feb 10 2021 at 19:39, Hikaru Nishida wrote:
> From: Hikaru Nishida <hikalium@...omium.org>
>
> This adds a sysfs interface /sys/kernel/boottime_adj to enable advancing
> CLOCK_BOOTTIME from the userspace without actual susupend/resume cycles.
>
> This gives a way to mitigate CLOCK_BOOTTIME divergence between guest
> and host on virtualized environments after suspend/resume cycles on
> the host.
>
> We observed an issue of a guest application that expects there is a gap
> between CLOCK_BOOTTIME and CLOCK_MONOTONIC after the device is suspended
> to detect whether the device went into suspend or not.
> Since the guest is paused instead of being actually suspended during the
> host's suspension, guest kernel doesn't advance CLOCK_BOOTTIME correctly
> and there is no way to correct that.
>
> To solve the problem, this change introduces a way to modify a gap
> between those clocks and align the timer behavior to host's one.

That's not a solution, that's a bandaid and just creating a horrible
user space ABI which we can't get rid off anymore.

The whole approach of virt vs. pausing and timekeeping is busted as I
pointed out several times before. Just papering over it with random
interfaces which fiddle with the timekeeping internals is not going to
happen.

Thanks,

        tglx