[Xen-devel] Re: lazy context switching

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 26 Aug 2005, at 17:38, Hollis Blanchard wrote:

> Hmmmmm. I think the basic problem is that in the exception handler we 
> don't usually know we will need this state. The exception is a debug 
> exception, where we know we will need it for the GDB stub.
> However, we also have a hypervisor-dedicated timer, HDEC (hypervisor 
> decrementer). Rather than using it as a plain tick which may or may 
> not cause a scheduler exception, we can use it to *always* mean a 
> context switch. In that case, we would always save the full state on 
> HDEC entry, because we know it will always cause a context switch. 
> Judging by set_ac_timer() callers, it seems that only the scheduler 
> really uses the Xen timer tick. If non-scheduler components start 
> using Xen-internal ticks, this approach wouldn't hold up (or rather, 
> it would start becoming less efficient).
Why not move the non-volatile save/restore into your context switch 
routine, rather than deferring it until you exit the hypervisor?
 -- Keir


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