linux-kernel - Re: [RFC PATCH v3 0/7] Add virtio

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Date: Tue, 19 Mar 2024 14:47:26 +0100
From: Peter Hilber <peter.hilber@...nsynergy.com>
To: David Woodhouse <dwmw2@...radead.org>, 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>,
 Parav Pandit <parav@...dia.com>
Subject: Re: [RFC PATCH v3 0/7] Add virtio_rtc module and related changes

While the virtio-comment list is not available, now also CC'ing Parav,
which may be interested in this virtio-rtc spec related discussion thread.

On 14.03.24 15:19, David Woodhouse wrote:
> 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).

We plan to add 

- leap second indication,

- UTC-to-TAI offset,

- clock smearing indication (including the noon-to-noon linear smearing
  variant which seems to be somewhat popular), and

- clock accuracy indication

to the initial spec and to the PoC implementation.

However, due to resource restrictions, we cannot ourselves add the
memory-mapped clock to the initial spec.

Everyone is very welcome to contribute the memory-mapped clock to the spec,
and I think it might then still make it to the initial version.

> 
> 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.

Agreed. That should be addressed by the above changes.

Best regards,

Peter