Re: [Xen-devel] [PATCH] Disable Xen PowerNow! support on Opteron 2nd gen and earlier processors

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 25/1/08 17:46, "John Levon" <levon@xxxxxxxxxxxxxxxxx> wrote:

> On Fri, Jan 25, 2008 at 05:19:53PM +0000, Stephen C. Tweedie wrote:
> 
>>> Seeing them at that frequency (less than one a minute) is not too serious.
>>> Perhaps we should get rid of the message, or reduce its priority.
>> 
>> They are most definitely serious --- gettimeofday() going backwards
>> breaks Unix semantics.  It breaks applications.  It _will_ corrupt
>> application data --- things like database rollback logs that rely on
>> timestamp monotonicity to order things correctly will break.
> 
> I thought Linux now enforced a monotonic clock the same way we were
> forced to - that is, globally synchronise and make sure we return last
> value + 1, even when the Xen clock goes backwards across CPUs, which it
> does regularly, powernow or not?

This is also true. Although in this case the backwards deltas of >100ms are
a bit extreme and will cause application-visible progress of time to stall
for that length of time. We've suggested some leads for Mark to look into,
so hopefully that will improve matters. Ultimately, the work being done to
put the C/P-state governors in Xen is the better approach, once that's
ready.

 -- Keir



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