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xen-ia64-devel
RE: [Xen-ia64-devel] SMP-guest status
>From: Tristan Gingold
>Sent: 2006年4月20日 21:22
>
>Le Jeudi 20 Avril 2006 05:47, Alex Williamson a écrit :
>> On Wed, 2006-04-19 at 11:38 +0200, Tristan Gingold wrote:
>> > + /* Fill a platform feature. ITC are (almost!) synchronized. .
>> > */
>> > + sal_feature->type = SAL_DESC_PLATFORM_FEATURE;
>> > + sal_feature->feature_mask =
>> > IA64_SAL_PLATFORM_FEATURE_ITC_DRIFT;
>
>> Is the jitter protection in the time interpolator sufficient for
>> ignoring this? Drift is really meant to indicate the ITCs are driven
>> from different time sources so may run at slightly different clock
>> frequencies. Seems we should only need to provide that flag to the
>> guest if the platform firmware set it. As long as the ITCs are nearly
>> synchronized, the jitter protection in the ITC interpolator will prevent
>> time from going backwards. This would then get rid of the change in
>> time.c. Thanks,
>The change in time.c is just to work around a kernel bug. Linux kernel
>requires at least an interpolator. [Hence I think there is no platform
>without ITC drift].
This is the sal platform feature printed on my tiger4 box:
SAL Platform features: BusLock
So there's no ITC drift provided. :-)
>
>I agree this area is more or less kludgy.
>
>The problems are:
>- if Xen doesn't set ITC_DRIFT, kernels tries to sync itc. It doesn't really
>work *yet* as itc cannot be set by domains. I will test this again.
So maybe you can add a clear comment that this is only a temporary
solution which will be replaced by real sal platform features later.
Thanks,
Kevin
>
>- If Xen sets ITC_DRIFT, it lies (a little or a lot).
>
>Tristan.
>
>
>
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