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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 8 Jun 2005, at 22:42, Kurt Garloff wrote:

> > I think that should work (with the obvious kernel_ prefix), but I've
> > appeneded what we've gone for.
> Not having a CPU manual close to my desk: What do we achieve by setting
> bit 3 (TS) in CR0? Why does it help to get the FPU back to a sane 
> state?
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.
 -- Keir


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