[Xen-devel] Re: About pit irq

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

Since the PIT advertises itself as a 32-bit timer, the platform overflow
logic will not trigger every 27ms. More like once an hour.

The issue with PIT is the 16-bit counter overflows within 10s of
milliseconds, and we have non-preemptible periods in Xen of longer than that
(e.g., during domain destruction). If we miss an overflow, time handling
gets totally screwed so we have to deal with PIT in hardirq context, not
softirq. We do it at 10ms because that's what we happen to always program
PIT for. Certainly in this specific case we could program it to tick slower.

 -- Keir

On 19/11/08 02:01, "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:

> Currently pit irq is enabled when using pit as clocksource or
> taking pit as broadcast means for deep C-state. I'm curious
> whether pit irq is really required for the 1st case. When pit
> is chosen as clocksource, 10ms periodical pit interrupt converts
> 16 counter into monotonic 32bit stamp, and then ~27ms platform
> overflow timer converts 32bit into 64bit stamp. Is the first
> conversion really necessary? platform overflow timer is chosen
> as 1/2 period, and it looks safe enough to convert from 16bit
> into 64bit stamp directly. Just one special bit to indicate
> whether counter counts down or up.
> 
> Thanks,
> Kevin



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