netdev - Re: [RFC PATCH v3 0/7] Add virtio_rtc module and related changes

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Message-ID: <9455F710-E38C-45DA-9883-EC034495ADEF@infradead.org>
Date: Thu, 14 Mar 2024 15:19:53 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Peter Hilber <peter.hilber@...nsynergy.com>, linux-kernel@...r.kernel.org,
 virtualization@...ts.linux.dev, virtio-dev@...ts.oasis-open.org,
 linux-arm-kernel@...ts.infradead.org, linux-rtc@...r.kernel.org,
 "virtio-comment@...ts.oasis-open.org" <virtio-comment@...ts.oasis-open.org>
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>,
 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>,
 "Ridoux, Julien" <ridouxj@...zon.com>, Cornelia Huck <cohuck@...hat.com>
Subject: Re: [RFC PATCH v3 0/7] Add virtio_rtc module and related changes

On 14 March 2024 11:13:37 CET, Peter Hilber <peter.hilber@...nsynergy.com> wrote:
>> To a certain extent, as long as the virtio-rtc device is designed to expose time precisely and unambiguously, it's less important if the Linux kernel *today* can use that. Although of course we should strive for that. Let's be...well, *unambiguous*, I suppose... that we've changed topics to discuss that though.
>> 
>
>As Virtio is extensible (unlike hardware), my approach is to mostly specify
>only what also has a PoC user and a use case.

If we get memory-mapped (X, Y, Z, ±x, ±y) I'll have a user and a use case on day one. Otherwise, as I said in my first response, I can go do that as a separate device and decide that virtio_rtc doesn't meet our needs (especially for maintaining accuracy over LM).

My main concern for virto_rtc is that we avoid *ambiguity*. Yes, I get that it's extensible but we don't want a v1.0 of the spec, implemented by various hypervisors, which still leaves guests not knowing what the actual time is. That would not be good. And even UTC without a leap second indicator has that problem.