xen-net{back, front}: Document multi-queue feature in netif.h

Document the multi-queue feature in terms of XenStore keys to be written
by the backend and by the frontend.
Signed-off-by: Andrew J. Bennieston <andrew.bennieston@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/xen/interface/io/netif.h

@@ -50,6 +50,59 @@
  * node as before.
  */
 
+/*
+ * Multiple transmit and receive queues:
+ * If supported, the backend will write the key "multi-queue-max-queues" to
+ * the directory for that vif, and set its value to the maximum supported
+ * number of queues.
+ * Frontends that are aware of this feature and wish to use it can write the
+ * key "multi-queue-num-queues", set to the number they wish to use, which
+ * must be greater than zero, and no more than the value reported by the backend
+ * in "multi-queue-max-queues".
+ *
+ * Queues replicate the shared rings and event channels.
+ * "feature-split-event-channels" may optionally be used when using
+ * multiple queues, but is not mandatory.
+ *
+ * Each queue consists of one shared ring pair, i.e. there must be the same
+ * number of tx and rx rings.
+ *
+ * For frontends requesting just one queue, the usual event-channel and
+ * ring-ref keys are written as before, simplifying the backend processing
+ * to avoid distinguishing between a frontend that doesn't understand the
+ * multi-queue feature, and one that does, but requested only one queue.
+ *
+ * Frontends requesting two or more queues must not write the toplevel
+ * event-channel (or event-channel-{tx,rx}) and {tx,rx}-ring-ref keys,
+ * instead writing those keys under sub-keys having the name "queue-N" where
+ * N is the integer ID of the queue for which those keys belong. Queues
+ * are indexed from zero. For example, a frontend with two queues and split
+ * event channels must write the following set of queue-related keys:
+ *
+ * /local/domain/1/device/vif/0/multi-queue-num-queues = "2"
+ * /local/domain/1/device/vif/0/queue-0 = ""
+ * /local/domain/1/device/vif/0/queue-0/tx-ring-ref = "<ring-ref-tx0>"
+ * /local/domain/1/device/vif/0/queue-0/rx-ring-ref = "<ring-ref-rx0>"
+ * /local/domain/1/device/vif/0/queue-0/event-channel-tx = "<evtchn-tx0>"
+ * /local/domain/1/device/vif/0/queue-0/event-channel-rx = "<evtchn-rx0>"
+ * /local/domain/1/device/vif/0/queue-1 = ""
+ * /local/domain/1/device/vif/0/queue-1/tx-ring-ref = "<ring-ref-tx1>"
+ * /local/domain/1/device/vif/0/queue-1/rx-ring-ref = "<ring-ref-rx1"
+ * /local/domain/1/device/vif/0/queue-1/event-channel-tx = "<evtchn-tx1>"
+ * /local/domain/1/device/vif/0/queue-1/event-channel-rx = "<evtchn-rx1>"
+ *
+ * If there is any inconsistency in the XenStore data, the backend may
+ * choose not to connect any queues, instead treating the request as an
+ * error. This includes scenarios where more (or fewer) queues were
+ * requested than the frontend provided details for.
+ *
+ * Mapping of packets to queues is considered to be a function of the
+ * transmitting system (backend or frontend) and is not negotiated
+ * between the two. Guests are free to transmit packets on any queue
+ * they choose, provided it has been set up correctly. Guests must be
+ * prepared to receive packets on any queue they have requested be set up.
+ */
+
 /*
  * "feature-no-csum-offload" should be used to turn IPv4 TCP/UDP checksum
  * offload off or on. If it is missing then the feature is assumed to be on.