[PDFv2] virtio-spec: 64 bit features, used/avail event

["Michael S. Tsirkin" <mst@redhat.com>]

People asked for a pdf for a new spec, so here it is:
http://userweb.kernel.org/~mst/virtio-spec-event-idx-v2.pdf

Guest and host implementation can be found here:
git://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git vhost-net-next-event-idx-v1
git://git.kernel.org/pub/scm/linux/kernel/git/mst/qemu-kvm.git virtio-net-event-idx-v1

Description reposted below:

I'm working on a patchset (to follow shortly)
that modified the notificatin hand-off in virtio to be basically
like Xen: each side published an index, the other side only triggers
an event when it crosses that index value
(Xen event indexes start at 1, ours start at 0 for
backward-compatiblity, but that's minor).

Especially for testing, it is very convenient to have
separate feature bits for this change in used and available
ring; since we've run out of bits in the 32 bit field,
I added another 32 bit and bit 31 enables that.

I started with using both flags and indexes in parallel,
but switched to doing either-or: this means we do
not need to tweak memory access ordering as index access just
replaces flags access.

A note on naming: the index replacing avail->flags is named
used_event, the index replacing used->flags is named
avail_event to stress the fact that these actually
point into the other side of the ring:
event is triggered when avail->idx == used->avail_event + 1
and when used->idx == avail->used_event + 1, respectively.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

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MST