[Xen-devel] Re: lazy context switching

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 25 Aug 2005, at 22:55, Hollis Blanchard wrote:

> Later on, if it turns out we are switching domains, we save/restore 
> all the
> state we can, then return to the exception handler which saves the old 
> set of
> nonvolatiles and loads the new one. Until that point, some domain 
> state is
> spread arbitrarily across our stack.
>
> That means that context_switch() cannot actually save all of @prev's 
> state to
> memory (and neither can __sync_lazy_execstate()) -- only by returning 
> all the
> way to assembly can we accomplish that.
>
> Thoughts?
What you need is a synchronisation point, visible to other CPUs, beyond 
which things like DOM0_GETVCPUCONTEXT can be sure to read consistent 
current state for the descheduled vcpu. See domain_sleep_sync() for the 
current way we ensure that state is committed to memory.
If you have a lot of register state, have you considered maintaining a 
Xen stack per VCPU? The context-switch interface already supports this, 
for ia64.
 -- Keir


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