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[Xen-devel] [RFC] Xen paravirtual watchdog device [0/4]

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Subject: [Xen-devel] [RFC] Xen paravirtual watchdog device [0/4]
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Fri, 1 Jun 2007 04:44:29 +0100
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Hi all,

As I've previously mentioned, I've been tinkering with a Xen-based virtual 
watchdog device and I think it's about time I put out some patches for 
discussion.  The principle of the watchdog device is to provide a simple, 
high-assurance watchdog to domains running on Xen.  The goal is to give 
better assurance than an in-kernel software watchdog.

The patches were developed for, but I've done what fixups I could 
see were necessary to make them apply against tip but I still need to test it 
works there correctly.  I'm primarily just putting these out here for people 
to eyeball and argue about, but if you do try them out please let me know how 
you get on.

Under Linux, a watchdog daemon is typically run in userspace.  This talks to 
the watchdog device driver through /dev/watchdog, which is responsible for 
resetting the timeout on the watchdog device whenever the daemon kicks it.  
The reliance on the userspace daemon ensures that scheduling, etc, is still 
occurring correctly in the system, and makes it easy for different 
functionality to be implemented on top of the basic watchdog feature.

The XenLinux watchdog driver sets up the in-Xen watchdog for the current 
domain using a simple hypercall interface.  The watchdog timer is implemented 
within Xen itself, isolating it from any guest bugs that might take out a 
normal software watchdog.  Standard interfaces to userspace are supported so 
that any watchdog-aware software should be able to interact with it without 

The patchset also includes some watchdog-awareness enhancements for the dom0 
toolset (more of these are possible in the future).

Comments, questions, abuse, anyone?


Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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