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Re: [Xen-devel] wget and Zope crashes on post-2.0.6 -testing

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Subject: Re: [Xen-devel] wget and Zope crashes on post-2.0.6 -testing
From: Robbie Dinn <robbie@xxxxxxxxxxxx>
Date: Fri, 10 Jun 2005 20:52:02 +0100
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Hi all

Keir Fraser wrote:

When set it causes a fault whenever the FPU is accessed. We use it to lazily initialise the FPU for the currently running process. At context-switch time we look at the process we are descheduling and, if it hasn;t used the FPU in its time slice, we don;t save FPU state and we don;t set the TS bit (because we assume it must be already set).

The last point is where we can fall down: if the TS bit in fact *isn;t* set, then we are screwed for all time. The kernel will never realise a process is using the FPU because we will never take the TS fault, because the TS bit is clear. Thus state doesn;t get saved/restored during context switch and the TS bit never gets set. So its a self perpetuating state once you're in it.

I have an end user question rather than a developers Q.

Say I have an xen machine with several domains, some with kernels
that have the FPU bug fix and some without. Can a domain with
the buggy kernel upset a domain with a bug free kernel?
Or does this just affect processes within one domain?

I might want to be a bit more hasty in upgrading all the kernels
if a buggy kernel/domain can upset a good kernel/domain.

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