Re: [Xen-devel] xen PIT timer

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 23 Sep 2005, at 20:49, Ryan Harper wrote:

> Why does Xen chose to use mode 0, on PIT CH2 for calculating how much
> time has passed rather than how Linux using PIT_CH0 in mode 2?  Are
> there some trade offs?
The way we track time in Xen is completely different to Linux. Linux 
tracks time off periodic interrupts from PIT_CH0: initially assuming 
each tick is 10ms but allowing that to be trimmed by adjtimex if ntpd 
is running.
Xen only uses PIT_CH0 as a fallback timer on uniprocessor systems with 
no local APIC: aprt from that it is basically unused. Xen doesn't rely 
on fixed periodic interrupts -- instead programming the local APIC 
timer to the next event of interest. We track time by assuming the PIT 
crystal is a precise 1.119380Mhz source: we can therefore measure the 
passage of time by reading how far PIT_CH2 has decremented and perform 
a multiplication. To more quickly estimate the passage of time, each 
CPU calibrates its local APIC oscillator to PIT_CH2 -- we can then 
estimate time-of-day with a RDTSC instruction + multiplication.
The main problem with using PIT as platform timer is that the counters 
are only 16 bits. We therefore periodically (every few 10s of ms) read 
the PIT_CH2 delta out into a wider 64-bit variable.
 -- Keir


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