Date: Sun, 27 Mar 2016 14:21:16 +0000 From: "Poul-Henning Kamp" <phk@phk.freebsd.dk> To: Konstantin Belousov <kostikbel@gmail.com> Cc: Bruce Evans <brde@optusnet.com.au>, John Baldwin <jhb@freebsd.org>, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org, "'rstone@freebsd.org'" <rstone@freebsd.org> Subject: Re: svn commit: r297039 - head/sys/x86/x86 Message-ID: <87078.1459088476@critter.freebsd.dk> In-Reply-To: <20160327130706.GA1741@kib.kiev.ua> References: <20160323075842.GX1741@kib.kiev.ua> <2922763.uITxoCVqGR@ralph.baldwin.cx> <20160324090917.GC1741@kib.kiev.ua> <20160325010649.H898@besplex.bde.org> <20160324162447.GD1741@kib.kiev.ua> <20160325060901.N2059@besplex.bde.org> <20160325084902.GH1741@kib.kiev.ua> <20160326021219.X911@besplex.bde.org> <20160326174210.GU1741@kib.kiev.ua> <20160327051601.S2318@besplex.bde.org> <20160327130706.GA1741@kib.kiev.ua>
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-------- In message <20160327130706.GA1741@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. >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. >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.) >> > 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. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.
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