linux-kernel - Re: [PATCH net-next v7] ptp: Add support for the AMZNC10C 'vmclock' device

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Message-ID: <20241028091256.1b0752b4@kernel.org>
Date: Mon, 28 Oct 2024 09:12:56 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: David Woodhouse <dwmw2@...radead.org>
Cc: Richard Cochran <richardcochran@...il.com>, 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>, "Chashper,
 David" <chashper@...zon.com>, "Mohamed Abuelfotoh, Hazem"
 <abuehaze@...zon.com>, Paolo Abeni <pabeni@...hat.com>, "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, Stephen Boyd <sboyd@...nel.org>, Thomas Gleixner
 <tglx@...utronix.de>, 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>, qemu-devel
 <qemu-devel@...gnu.org>, Simon Horman <horms@...nel.org>
Subject: Re: [PATCH net-next v7] ptp: Add support for the AMZNC10C 'vmclock'
 device

On Sat, 19 Oct 2024 18:49:24 +0100 David Woodhouse wrote:
> > Yes please and thank you! We gotta straighten it out before 
> > the merge window.  
> 
> Hm, as I (finally) come to do that, I realise that many of the others
> defined in drivers/ptp/Kconfig are also 'default y'. Which is only
> really 'default PTP_1588_CLOCK' in practice since they all depend on
> that.

AFAICT nothing defaulted to enabled since 2017, so I'd chalk it up
to us getting better at catching mistakes over time.

> Most importantly, PTP_1588_CLOCK_KVM is 'default y'. And that one is
> fundamentally broken (at least in the presence of live migration if
> guests care about their clock suddenly being wrong) which is why it's
> being superseded by the new VMCLOCK thing. We absolutely don't want to
> leave the _KVM one enabled by default and not its _VMCLOCK replacement.

You can default to .._CLOCK_KVM, and provide the explanation in
the commit message and Kconfig help.
Or if you feel strongly even make CLOCK_KVM depend on the new one?

> Please advise... I suspect the best answer is to leave it as it is? 

I'd really rather not. Linus has complained to us about Kconfig symbols
appearing / getting enabled by default multiple times in the past.

Sorry for the delay, vacation time.