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

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Date: Tue, 25 Jun 2024 22:48:22 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Thomas Gleixner <tglx@...utronix.de>, 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, 2024-06-25 at 23:34 +0200, Thomas Gleixner wrote:
> 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.

Ooh, interesting. Got a reference to that please?

>  Can we please not expose an ad hoc interface for that?

Absolutely. I'm explicitly trying to intercept the virtio-rtc
specification here, to *avoid* having to do anything ad hoc.

Note that this is a "vDSO-style" interface from hypervisor to guest via
a shared memory region, not necessarily an actual vDSO.

But yes, it *is* intended to be exposed to userspace, so that userspace
can know the *accurate* time without a system call, and know that it
hasn't been perturbed by live migration.

> 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.

I have not heard any news, although now I'm making inferences.

Wishing you the best!

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