Re: Wait queue support for 4.3

[Keir Fraser <keir@xen.org>]

On 12/10/2012 16:38, "Andres Lagar-Cavilla" <andreslc@gridcentric.ca> wrote:

> 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
> passes
> 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
> perform.
>  
> The current maintainer (effectively) for wait queues is Keir. Keir, any ideas
> on a schedule for the cleanup?

I maintain the wait-queue mechanism, but not every (potential) user of it!
The only use-case above that might fall into my domain is 3, I think.

 -- Keir

> Thanks
> Andres
>