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

 * 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.