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RE: [Xen-devel] [PATCH] per-cpu timer changes

  • To: "Don Fry" <brazilnut@xxxxxxxxxx>, <Xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
  • Date: Tue, 24 May 2005 02:20:36 +0100
  • Delivery-date: Tue, 24 May 2005 01:20:00 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AcVf+J+EJOz9VzpmQOmffaVUFdqUmwAAyRPA
  • Thread-topic: [Xen-devel] [PATCH] per-cpu timer changes

> Here are some patches to help xen run on a system where the 
> CPUs are not synchronized so that the tsc counters drift.  
> Without this patch I get lots of 'time went backwards' 
> messages in the linux logs.  I still get an occasional 
> message, but not the number I did previously.


This is looking good. To help other people review the patch, it might be
a good idea to post some of the design discussion we had off list as I
think the approach will be new to most people. (Perhaps put some of the
text in a comment in the hypervisor interface).

As regards the time going backwards messages, if you're seeing small -ve
deltas, I'm not surprised -- you need to round to some precision as we
won't be nanosecond accurate. Experience suggests we'll be good for a
few 10's of ns with any kind of decent crystal. We could round to e.g.
512ns or 1024ns to make sure.


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