Re: [Xen-devel] Workaround for buggy PIT

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 15 Mar 2006, at 19:52, Tomas Kopal wrote:

> I was printing out real diff values (detecting min and max over periods
> of time) and it varied about 40% around the latch value. I didn't want
> to get too many false positives, so I set it to double the expected
> value. As the problematic values tend to be quite high, I think this is
> a safe threshold.
40% range is huge, given that Xen disables interrupts only for very 
short periods of time.
If you think that the timer ends up corrupting its count value, but 
continues counting in the mode we originally programmed it to, there 
would be no need for your patch to reprogram the timer. We could just 
clamp diff and let the timer continue to free-run from whatever value 
it corrupted itself to. Would that simpler patch, with no 
reprogramming, work for you?
I agree with your suspicion that this may be a channel-0 problem. The 
40% value range points at some serious weirdness.
As for other timer problems -- the really common one (latched reads do 
not latch, so you get inconsistent 16-bit reads) don't long-term affect 
our time stability. We end up out by a few hundred microseconds on that 
read of the clock, but the 16-bit timer value doesn't wrap or anything 
really bad like that, so we can recover. Phew. :-)
 -- Keir


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