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

[David Woodhouse <dwmw2@infradead.org>]

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On Wed, 2024-03-13 at 13:58 +0100, Alexandre Belloni wrote:
> The TSC or whatever CPU counter/clock that is used to keep the system
> time is not an RTC, I don't get why it has to be exposed as such to the
> guests. PTP is fine and precise, RTC is not.

Ah, I see. But the point of the virtio_rtc is not really to expose that
CPU counter. The point is to report the wallclock time, just like an
actual RTC. The real difference is the *precision*.

The virtio_rtc device has a facility to *also* expose the counter,
because that's what we actually need to gain that precision...

Applications don't read the RTC every time they want to know what the
time is. These days, they don't even make a system call; it's done
entirely in userspace mode. The kernel exposes some shared memory,
essentially saying "the counter was X at time Y, and runs at Z Hz".
Then applications just read the CPU counter and do some arithmetic.

As we require more and more precision in the calibration, it becomes
important to get *paired* readings of the CPU counter and the wallclock
time at precisely the same moment. If the guest has to read one and
then the other, potentially taking interrupts, getting preempted and
suffering steal/SMI time in the middle, that introduces an error which
is increasingly significant as we increasingly care about precision.

Peter's proposal exposes the pairs of {X,Y} and leaves *all* the guest
kernels having to repeat readings over time and perform the calibration
as the underlying hardware oscillator frequency (Z) drifts with
temperature. I'm trying to get him to let the hypervisor expose the
calibrated frequency Z too. Along with *error* bounds for ±δX and ±δZ.
Which aside from reducing the duplication of effort, will *also* fix
the problem of live migration where *all* those things suffer a step
change and leave the guest with an inaccurate clock but not knowing it.

But that part isn't relevant to the RTC, as you say. RTC doesn't care
about that level of precision; it's just what the system uses to know
roughly what year it is, when it powers up. (And isn't even really
trusted for that, which is a large part of why I added the
X509_V_FLAG_NO_CHECK_TIME flag to OpenSSL, so Secure Boot doesn't break
when the RTC is catastrophically wrong :)

If you're asking why patch 7/7 in Peter's series exists to expose the
virtio clock through RTC, and you're not particularly interested in the
first six, I suppose that's a fair question. As is the question of "why
is it called virtio_rtc not virtio_ptp?". 

But let me turn it around: if the kernel has access to this virtio
device and *not* any other RTC, why *wouldn't* the kernel use the time
from it? The fact that it can optionally *also* provide paired readings
with the CPU counter doesn't actually *hurt* for the RTC use case, does
it?




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