From: Rusty Russell <rusty@rustcorp.com.au>
Date: Thu, 12 Jan 2012 05:14:43 +0000 (+1030)
Subject: virtio: expose added descriptors immediately.
X-Git-Tag: v3.3-rc1~85^2~17
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=ee7cd8981e15bcb365fc762afe3fc47b8242f630;p=pandora-kernel.git

virtio: expose added descriptors immediately.

A virtio driver does virtqueue_add_buf() multiple times before finally
calling virtqueue_kick(); previously we only exposed the added buffers
in the virtqueue_kick() call.  This means we don't need a memory
barrier in virtqueue_add_buf(), but it reduces concurrency as the
device (ie. host) can't see the buffers until the kick.

In the unusual (but now possible) case where a driver does add_buf()
and get_buf() without doing a kick, we do need to insert one before
our counter wraps.  Otherwise we could wrap num_added, and later on
not realize that we have passed the marker where we should have
kicked.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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