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

[David Woodhouse]

On Tue, 2024-03-19 at 14:47 +0100, Peter Hilber wrote:
> 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@xxxxxxxxxxxxxxx> 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.

Sounds good, thanks! I look forward to seeing the new revision. I'm
hoping Julien can give feedback on the clock accuracy parts.

> 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.

Makes sense. That is my primary target, so I'm *hoping* we can converge
and get that into your initial spec, otherwise for expediency I'm going
to have to define an ACPI or DT or PCI device of our own and expose the
memory region through that instead.

(Even if I have to do that in the short term to stop the bleeding with
customers' clocks and live migration, I'd still aspire to migrate to a
virtio_rtc version of it in future)

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