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

From: David Woodhouse <dwmw2@infradead.org>
To: Peter Hilber <peter.hilber@opensynergy.com>,
	linux-kernel@vger.kernel.org, virtualization@lists.linux.dev,
	virtio-dev@lists.oasis-open.org,
	linux-arm-kernel@lists.infradead.org, linux-rtc@vger.kernel.org,
	"virtio-comment@lists.oasis-open.org"
	<virtio-comment@lists.oasis-open.org>
Cc: "Christopher S. Hall" <christopher.s.hall@intel.com>,
	Jason Wang <jasowang@redhat.com>,
	John Stultz <jstultz@google.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	netdev@vger.kernel.org,
	Richard Cochran <richardcochran@gmail.com>,
	Stephen Boyd <sboyd@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
	Marc Zyngier <maz@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Daniel Lezcano <daniel.lezcano@linaro.org>,
	Alessandro Zummo <a.zummo@towertech.it>,
	Alexandre Belloni <alexandre.belloni@bootlin.com>,
	"Ridoux, Julien" <ridouxj@amazon.com>,
	Cornelia Huck <cohuck@redhat.com>
Subject: Re: [RFC PATCH v3 0/7] Add virtio_rtc module and related changes
Date: Thu, 14 Mar 2024 15:19:53 +0100	[thread overview]
Message-ID: <9455F710-E38C-45DA-9883-EC034495ADEF@infradead.org> (raw)
In-Reply-To: <2eb5a616-eeb3-446a-85fd-fff376c15f55@opensynergy.com>

On 14 March 2024 11:13:37 CET, Peter Hilber <peter.hilber@opensynergy.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.

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