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Xen 3.0 Virtualization Interface Guide
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4.1 Shared info page

The shared_info_t is accessed at run time by both Xen and the guest OS. It is used to pass information relating to the virtual CPU and virtual machine state between the OS and the hypervisor.

The structure is declared in xen/include/public/xen.h:

typedef struct shared_info {
    vcpu_info_t vcpu_info[MAX_VIRT_CPUS];

    /*
     * 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.
     */
    unsigned long evtchn_pending[sizeof(unsigned long) * 8];
    unsigned long evtchn_mask[sizeof(unsigned long) * 8];

    /*
     * Wallclock time: updated only by control software. Guests should base
     * their gettimeofday() syscall on this wallclock-base value.
     */
    uint32_t wc_version;      /* Version counter: see vcpu_time_info_t. */
    uint32_t wc_sec;          /* Secs  00:00:00 UTC, Jan 1, 1970.  */
    uint32_t wc_nsec;         /* Nsecs 00:00:00 UTC, Jan 1, 1970.  */

    arch_shared_info_t arch;

} shared_info_t;

vcpu_info
An array of vcpu_info_t structures, each of which holds either runtime information about a virtual CPU, or is ``empty'' if the corresponding VCPU does not exist.
evtchn_pending
Guest-global array, with one bit per event channel. Bits are set if an event is currently pending on that channel.
evtchn_mask
Guest-global array for masking notifications on event channels.
wc_version
Version counter for current wallclock time.
wc_sec
Whole seconds component of current wallclock time.
wc_nsec
Nanoseconds component of current wallclock time.
arch
Host architecture-dependent portion of the shared info structure.

Xen 3.0 Virtualization Interface Guide
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  Published under the terms of the GNU General Public License Design by Interspire