linux-kernel - Re: [RFC PATCH v2] ptp: Add vDSO-style vmclock support

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Date: Tue, 25 Jun 2024 23:34:24 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: David Woodhouse <dwmw2@...radead.org>, Peter Hilber
 <peter.hilber@...nsynergy.com>, linux-kernel@...r.kernel.org,
 virtualization@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
 linux-rtc@...r.kernel.org, "Ridoux,
 Julien" <ridouxj@...zon.com>, virtio-dev@...ts.linux.dev, "Luu, Ryan"
 <rluu@...zon.com>
Cc: "Christopher S. Hall" <christopher.s.hall@...el.com>, Jason Wang
 <jasowang@...hat.com>, John Stultz <jstultz@...gle.com>, "Michael S.
 Tsirkin" <mst@...hat.com>, netdev@...r.kernel.org, Richard Cochran
 <richardcochran@...il.com>, Stephen Boyd <sboyd@...nel.org>, Xuan Zhuo
 <xuanzhuo@...ux.alibaba.com>, Marc
 Zyngier <maz@...nel.org>, Mark Rutland <mark.rutland@....com>, Daniel
 Lezcano <daniel.lezcano@...aro.org>, Alessandro Zummo
 <a.zummo@...ertech.it>, Alexandre Belloni <alexandre.belloni@...tlin.com>
Subject: Re: [RFC PATCH v2] ptp: Add vDSO-style vmclock support

On Tue, Jun 25 2024 at 20:01, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
>
> The vmclock "device" provides a shared memory region with precision clock
> information. By using shared memory, it is safe across Live Migration.
>
> Like the KVM PTP clock, this can convert TSC-based cross timestamps into
> KVM clock values. Unlike the KVM PTP clock, it does so only when such is
> actually helpful.
>
> The memory region of the device is also exposed to userspace so it can be
> read or memory mapped by application which need reliable notification of
> clock disruptions.

There is effort underway to expose PTP clocks to user space via
VDSO. Can we please not expose an ad hoc interface for that?

As you might have heard the sad news, I'm not feeling up to the task to
dig deeper into this right now. Give me a couple of days to look at this
with working brain.

Thanks,

        tglx