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

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Date:   Wed, 10 Feb 2021 11:48:15 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Hikaru Nishida <hikalium@...omium.org>
Cc:     linux-kernel@...r.kernel.org, suleiman@...gle.com,
        Alexander Graf <graf@...zon.com>,
        Andra Paraschiv <andraprs@...zon.com>,
        Masahiro Yamada <masahiroy@...nel.org>
Subject: Re: [RFC PATCH 2/2] drivers/virt: introduce CLOCK_BOOTTIME
 adjustment sysfs interface driver

On Wed, Feb 10, 2021 at 07:39:08PM +0900, 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.
> 
> Signed-off-by: Hikaru Nishida <hikalium@...omium.org>
> ---
> 
>  drivers/virt/Kconfig        |  9 ++++++
>  drivers/virt/Makefile       |  1 +
>  drivers/virt/boottime_adj.c | 57 +++++++++++++++++++++++++++++++++++++
>  3 files changed, 67 insertions(+)
>  create mode 100644 drivers/virt/boottime_adj.c

No Documentation/ABI/ update for your new sysfs file?  Please fix...