On 13/03/2024 10:45:54+0100, Peter Hilber wrote:

quoted

Exposing UTC as the only clock reference is bad enough; when leap
seconds happen there's a whole second during which you don't *know*
which second it is. It seems odd to me, for a precision clock to be
deliberately ambiguous about what the time is!

Just to be clear, the device can perfectly expose only a TAI reference
clock (or both UTC and TAI), the spec is just completely open about this,
as it tries to work for diverse use cases.

quoted

But if the virtio-rtc clock is defined as UTC and then expose something
*different* in it, that's even worse. You potentially end up providing
inaccurate time for a whole *day* leading up to the leap second.

I think you're right that leap second smearing should be addressed. At
the very least, by making it clear that the virtio-rtc clock which
advertises UTC shall be used *only* for UTC, never UTC-SLS or any other
yet-to-be-defined variant.

Agreed.

quoted

Please make it explicit that any hypervisor which wants to advertise a
smeared clock shall define a new type which specifies the precise
smearing algorithm and cannot be conflated with the one you're defining
here.

I will add a requirement that the UTC clock can never have smeared/smoothed
leap seconds.

I think that not every vendor would bother to first add a definition of a
smearing algorithm. Also, I think in some cases knowing the precise
smearing algorithm might not be important (when having the same time as the
hypervisor is enough and accuracy w.r.t. actual time is less important).

So maybe I should add a VIRTIO_RTC_CLOCK_UTC_SMEARED clock type, which for
now could catch every UTC-like clock which smears/smoothes leap seconds,
where the vendor cannot be bothered to add the smearing algorithm to spec
and implementations.

I still don't know anything about virtio but under Linux, an RTC is
always UTC (or localtime when dual booting but let's not care) and never
accounts for leap seconds. Having an RTC and RTC driver behaving
differently would be super inconvenient. Why don't you leave this to
userspace?

I guess I'm still questioning whether this is the correct interface to
expose the host system time instead of an actual RTC.

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com