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[Xen-devel] Wait queue support for 4.3

During the last Xen Summit there were informal discussions about the status of 
wait queues in the hypervisor.

To recap:
1. Wait queues are used in mem event, when events generated by a vcpu overflow 
the ring size
2. We would like to use wait queues when the hypervisor needs a paged out frame 
(say for hvm_copy)
3. We would like to use wait queues to avoid the two decoupled mmio emulation 
4. We would like to use wait queues when the hypervisor needs write access to a 
shared frame (say for hvm copy), and unsharing temporarily fails with ENOMEM.

Conceivably more uses for wait queues may come down the line.

Use-cases 2. and 4. were left out of the time frame of 4.2, because a vcpu 
cannot go to sleep on a wait queue while holding a spinlock, and such 
situations would frequently arise. Preliminary patches from Tim Deegan have 
floated on the list 
(http://lists.xen.org/archives/html/xen-devel/2012-02/msg02133.html). We would 
like this functionality to be present on the mm side for 4.3, and then proceed 
to remove the "thinking" that consumers of the p2m interface now need to 
The current maintainer (effectively) for wait queues is Keir. Keir, any ideas 
on a schedule for the cleanup?


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