+/*D:200
+ * Lguest I/O
+ *
+ * The lguest I/O mechanism is the only way Guests can talk to devices. There
+ * are two hypercalls involved: SEND_DMA for output and BIND_DMA for input. In
+ * each case, "struct lguest_dma" describes the buffer: this contains 16
+ * addr/len pairs, and if there are fewer buffer elements the len array is
+ * terminated with a 0.
+ *
+ * I/O is organized by keys: BIND_DMA attaches buffers to a particular key, and
+ * SEND_DMA transfers to buffers bound to particular key. By convention, keys
+ * correspond to a physical address within the device's page. This means that
+ * devices will never accidentally end up with the same keys, and allows the
+ * Host use The Futex Trick (as we'll see later in our journey).
+ *
+ * SEND_DMA simply indicates a key to send to, and the physical address of the
+ * "struct lguest_dma" to send. The Host will write the number of bytes
+ * transferred into the "struct lguest_dma"'s used_len member.
+ *
+ * BIND_DMA indicates a key to bind to, a pointer to an array of "struct
+ * lguest_dma"s ready for receiving, the size of that array, and an interrupt
+ * to trigger when data is received. The Host will only allow transfers into
+ * buffers with a used_len of zero: it then sets used_len to the number of
+ * bytes transferred and triggers the interrupt for the Guest to process the
+ * new input. */