Re: [Xen-devel] Re: [PATCH 2/2] xen/gnt{dev, alloc}: reserve event channels for notify

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2011-10-25 at 20:41 +0100, Daniel De Graaf wrote:
> 
> >> Hmm. Perhaps have a magic value for refcount (-1?) that indicates
> evtchn_get is not
> >> available. That would become the default value of refcnt, and
> evtchn.c would then
> >> use evtchn_make_refcounted() to change the refcount to 1 and allow
> _get/_put to work.
> > 
> > How would that work when the IRQ subsystem (so everything is setup
> in the kernel)
> > gets an event? Would the refcount be for that -1.. oh. You would
> only set
> > the refcnt when the _get/_put calls are made and not when in-kernel
> calls to setup> IRQs are done?
> > 
> 
> Right. The reference count would be a dual-purpose field indicating if
> the event channel is kernel-internal (value -1) or userspace-visible
> (reference count > 0). New event channels would start out at -1, and
> evtchn.c would change them to 1. 

Is there any way that the reference count could be made part of the
datastructures associated with the /dev/xen/evtchn driver instead of the
core evtchn.c stuff? That wouldreduce the chance of current or futures
users getting something wrong.

Ian.


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