Re: [Xen-devel] context switch

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 28 Mar 2006, at 18:33, Ky Srinivasan wrote:

>  Looking more at the generic Linux CS code, saving the selector values
> of the outgoing context and setting the segment registers values to 
> zero
> in prepare_arch_switch() we think deals with the problem I have listed
> below (thanks to Jan for pointing this out).
That's what our own trees do already.

> While this expensive trick
> may solve this problem, a simpler solution perhaps might be to have an
> efficient  mechanism for the guest to manage hypervisor preemptions.
Why is it expensive? The updates to zero only happen if the previous 
selector value was non-zero, which is usually not the case for 64-bit 
apps.
Things should work okay even without the zeroing, by the way. It just 
avoids an unnecessary failsafe callback into the guest kernel. I fixed 
the failsafe handler for x86/64 earlier today.
 -- Keir


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