|
xen-devel
Re: [Xen-devel] xen PIT timer
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
_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel
|
|
|
Home