On Sun, 27 Mar 2016, Poul-Henning Kamp wrote:
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In message <20160327130706.ga1...@kib.kiev.ua>, Konstantin Belousov writes:
I haven't seen a single (hardware) system since I started measuring
this 15-20 yeas ago on which the i8524 is not fed from the same clock
as the TSC.
A very important and relevant detail here is that the *only*
clock/counter which has a standardized frequency *is* the i8254
counter.
Not even the ACPI timer?
RTC needs its own dedicated crystal.
The RTC *crystal* may not even exist, only the RTC itself which may
be driven by gremlins on a threadmill for all we care.
Besides, getting hold of *precise* timing from the RTC is a nightmare,
it's not meant to be used for that.
It can give a precision of 1/32K seconds fairly easily (but with lots of
overhead) by interrupting at 32 KHz. 1KHz is sometimes used for profiling.
I was not able to find any specifications for allowed jitter in PCH PLLs,
but I would expect that above IDT chips have much better stability than
something in overheating PCH.
Jitter requirements are pretty tight for anything you're going to
PLL into the GHz range, but any quartz crystal is going to have much
better phase-noise spec than anything we can measure in the digital
noise of a modern computer.
(Often the PLL chips modulate the quartz to spread out EMI spurs,
look for "spread-spectrum" settings.)
Jitter is what I'm most worried about for virtual systems.
Jitter is quite good for a hardware RTC. I use the RTC for PPS mainly
to determine its quality. Its hardware jitter is insignificant compared
with the jitter from interrupt latency.
some hypervisors start offering modes where old ISA peripherals
are not emulated,
How do they expect people to run MS Flight Simulator then ? :-)
I think we have no choice but do something for ISA/LPC-less configurations.
We could even trust CPU report about its frequency as the last resort.
Usually the errors will be magnificient, so a trivial sanity-check will
catch them. Don't leave home without it.
But then you have the problem of handling them. There might be just 1
timecounter source that is emulated correctly, but you don't know which
it is. There might be none.
Bruce
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