linux-kernel - Re: [PATCH net-next v4] ptp: add Alibaba CIPU PTP clock driver

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Message-ID: <91008c3cd2502b3726992b0f490aac54a1efa186.camel@infradead.org>
Date: Thu, 21 Aug 2025 21:12:48 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Richard Cochran <richardcochran@...il.com>, Jakub Kicinski
 <kuba@...nel.org>
Cc: Wen Gu <guwen@...ux.alibaba.com>, andrew+netdev@...n.ch, 
 davem@...emloft.net, edumazet@...gle.com, pabeni@...hat.com, 
 xuanzhuo@...ux.alibaba.com, dust.li@...ux.alibaba.com,
 netdev@...r.kernel.org,  linux-kernel@...r.kernel.org, Thomas Gleixner
 <tglx@...utronix.de>
Subject: Re: [PATCH net-next v4] ptp: add Alibaba CIPU PTP clock driver

On Thu, 2025-08-21 at 13:05 -0700, Richard Cochran wrote:
> On Fri, Aug 15, 2025 at 11:38:14AM -0700, Jakub Kicinski wrote:
> 
> > Maybe it's just me, but in general I really wish someone stepped up
> > and created a separate subsystem for all these cloud / vm clocks.
> > They have nothing to do with PTP. In my mind PTP clocks are simple HW
> > tickers on which we build all the time related stuff. While this driver
> > reports the base year for the epoch and leap second status via sysfs.
> 
> Yeah, that is my feeling as well.

Agreed. While vmclock is presented as a PTP clock for compatibility,
it's more than that because it can do better than simply giving a
single precise point in time. It actually tells the guest the precise
relationship between the hardware counter and real time, much like the
private data structure exposed from the kernel to vDSO gettimeofday.

We should work on having the kernel consume that *directly* and feed
its CLOCK_REALTIME with it. Having hundreds of guests on the same host
all recalculate the same thing based on a set of points in time,
obtained through /dev/ptpX or otherwise, is just daft.

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