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 21:42 +0100, Daniel De Graaf wrote:
> On 10/25/2011 04:27 PM, Ian Campbell wrote:
> > 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.
> 
> This would require that the gntdev and gntalloc modules have a dependency
> on the evtchn module, with the evtchn_{get,put} functions moved into that
> module.

Don't they effectively have that already, since the only evtchns you can
use with them have come from that module?

>  The only other way would be to add a refcount maintenance
> function pointer to the IRQ data structure, which seems like it would lead
> to more problems than a simple reference count.
> 



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