16384 /* * A domain can create "event channels" on which it can send and receive * asynchronous event notifications. There are three classes of event that * are delivered by this mechanism: * 1. Bi-directional inter- and intra-domain connections. Domains must * arrange out-of-band to set up a connection (usually by allocating * an unbound 'listener' port and avertising that via a storage service * such as xenstore). * 2. Physical interrupts. A domain with suitable hardware-access * privileges can bind an event-channel port to a physical interrupt * source. * 3. Virtual interrupts ('events'). A domain can bind an event-channel * port to a virtual interrupt source, such as the virtual-timer * device or the emergency console. * * Event channels are addressed by a "port index". Each channel is * associated with two bits of information: * 1. PENDING -- notifies the domain that there is a pending notification * to be processed. This bit is cleared by the guest. * 2. MASK -- if this bit is clear then a 0->1 transition of PENDING * will cause an asynchronous upcall to be scheduled. This bit is only * updated by the guest. It is read-only within Xen. If a channel * becomes pending while the channel is masked then the 'edge' is lost * (i.e., when the channel is unmasked, the guest must manually handle * pending notifications as no upcall will be scheduled by Xen). * * To expedite scanning of pending notifications, any 0->1 pending * transition on an unmasked channel causes a corresponding bit in a * per-vcpu selector word to be set. Each bit in the selector covers a * 'C long' in the PENDING bitfield array. */