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Re: [Xen-devel] context switch

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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