On Tue, Mar 21, 2006 at 05:30:23PM -0500, Jed Davis wrote:
> Brett Lymn <blymn@baesystems.com.au> writes:
> 
> > You may want to look at some of the audio drivers which allow you to
> > mmap a buffer into userland.  From memory they handle the
> > userland/kernel interaction by updating the head/tail pointers in a
> > circular queue, userland updates one pointer and the kernel updates
> > the other... when they pointers are equal the queue is empty.
> > Unfortunately, it still needs an ioctl to kick the process along but
> > the bulk data transfer is handled more efficiently.
> 
> Virtual devices under Xen work a lot like that -- shared mapping of a
> ring buffer, incrementing produces/consumer indices, and virtual
> interrupts used by each side to kick the other.  The relevant code,
> alas, isn't the clearest thing out there, and requirement of memory
> barriers (since the producer and consumer domains could be running
> concurrently on different CPUs in a MP machine) probably doesn't help.

This has improved in Xen3, where a generic ring API has been designed, and
the rings are accessed though macros. This makes things clearer.

-- 
Manuel Bouyer <bouyer@antioche.eu.org>
     NetBSD: 26 ans d'experience feront toujours la difference
--