[Xen-devel] Re: lazy context switching

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 26 Aug 2005, at 18:40, Hollis Blanchard wrote:

> context_switch() will be called from somewhere beneath c_handler(). At 
> that point, the original nonvolatiles will have been saved across many 
> stack frames (starting with c_handler()'s), so we really are unable to 
> access them at this point. However, we trust that by the time we get 
> back to the exception handler, the original nonvolatiles will have 
> been restored off all those stack frames.
Hmmmm... Anyone interested in the state of a paused vcpu will call 
sync_vcpu_execstate() after descheduling the vcpu. That is an entirely 
arch-specific function that you can define to wait until the 
non-volatile registers are safely saved to memory.
Maybe you could add a flag to the arch-specific portion of the vcpu 
structure that gets set after the non-volatile registers are saved to 
memory and cleared when they are restored to active use. Then 
sync_vcpu_execstate() can spin on that flag waiting for it to be 
non-zero.
 -- Keir


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