svn commit: r297039 - head/sys/x86/x86

[Bruce Evans]

On Sun, 27 Mar 2016, Poul-Henning Kamp wrote:

> --------
> In message <20160327130706.GA1741 at 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