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xen-devel
Re: [Xen-devel] context switch
>>> On Tue, Mar 28, 2006 at 12:40 pm, in message
<af304b148f305da9c9b21ff4622322a3@xxxxxxxxxxxx>, Keir Fraser
<Keir.Fraser@xxxxxxxxxxxx> wrote:
> 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.
The expense I was referring to is the selector loads (to zero them
out). The prepare_arch_switch() is also used on the 32 bit side as well
(for fs and gs).
>
> 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
K. Y
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