Re: [Xen-devel] [RFC PATCH V3 21/22] Only allow extended event channel on Dom0 and driver domains

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, 2013-02-28 at 12:43 +0000, David Vrabel wrote:
> On 27/02/13 14:34, Wei Liu wrote:
> > For non-Dom0 domains, add a flag to indicate whether it can use extended 
> > event
> > channel, admins can specify this flag when creating a driver domain.
> > 
> > The rationale behide this option is, a) extended event channel will
> > consume global mapping space in Xen, b) likely that only Dom0 and driver
> > domains need this.
> 
> I would prefer the toolstack to limit the max port that the guest can
> use (see the EVTCHNOP_set_limit sub-op I proposed), than this single bit
> (which isn't useful for the FIFO-based proposal).
> 

I'm not very keen on adding that EVTCHNOP_set_limit now, because:

      * limiting number of event channels in 2/3-level brings no
        significant improvement.
      * you would need to way to notify guest kernel its limit (be
        another OP or via Xenstore), because in any case a guest uses
        user space evtchn driver it would need to know the limit.

In any case, the purpose of this option is to indicate whether the guest
can use *any* of the extended ABIs. A better naming should be
evtchn_extended_allowed, which should default to 1 in FIFO-based ABIs.


Wei.

> David



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