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-
-Device Specification for Inter-VM shared memory device
-------------------------------------------------------
-
-The Inter-VM shared memory device is designed to share a memory region (created
-on the host via the POSIX shared memory API) between multiple QEMU processes
-running different guests. In order for all guests to be able to pick up the
-shared memory area, it is modeled by QEMU as a PCI device exposing said memory
-to the guest as a PCI BAR.
-The memory region does not belong to any guest, but is a POSIX memory object on
-the host. The host can access this shared memory if needed.
-
-The device also provides an optional communication mechanism between guests
-sharing the same memory object. More details about that in the section 'Guest to
-guest communication' section.
-
-
-The Inter-VM PCI device
------------------------
-
-From the VM point of view, the ivshmem PCI device supports three BARs.
-
-- BAR0 is a 1 Kbyte MMIO region to support registers and interrupts when MSI is
-  not used.
-- BAR1 is used for MSI-X when it is enabled in the device.
-- BAR2 is used to access the shared memory object.
-
-It is your choice how to use the device but you must choose between two
-behaviors :
-
-- basically, if you only need the shared memory part, you will map BAR2.
-  This way, you have access to the shared memory in guest and can use it as you
-  see fit (memnic, for example, uses it in userland
-  http://dpdk.org/browse/memnic).
-
-- BAR0 and BAR1 are used to implement an optional communication mechanism
-  through interrupts in the guests. If you need an event mechanism between the
-  guests accessing the shared memory, you will most likely want to write a
-  kernel driver that will handle interrupts. See details in the section 'Guest
-  to guest communication' section.
-
-The behavior is chosen when starting your QEMU processes:
-- no communication mechanism needed, the first QEMU to start creates the shared
-  memory on the host, subsequent QEMU processes will use it.
-
-- communication mechanism needed, an ivshmem server must be started before any
-  QEMU processes, then each QEMU process connects to the server unix socket.
-
-For more details on the QEMU ivshmem parameters, see qemu-doc documentation.
-
-
-Guest to guest communication
-----------------------------
-
-This section details the communication mechanism between the guests accessing
-the ivhsmem shared memory.
-
-*ivshmem server*
-
-This server code is available in qemu.git/contrib/ivshmem-server.
-
-The server must be started on the host before any guest.
-It creates a shared memory object then waits for clients to connect on a unix
-socket. All the messages are little-endian int64_t integer.
-
-For each client (QEMU process) that connects to the server:
-- the server sends a protocol version, if client does not support it, the client
-  closes the communication,
-- the server assigns an ID for this client and sends this ID to him as the first
-  message,
-- the server sends a fd to the shared memory object to this client,
-- the server creates a new set of host eventfds associated to the new client and
-  sends this set to all already connected clients,
-- finally, the server sends all the eventfds sets for all clients to the new
-  client.
-
-The server signals all clients when one of them disconnects.
-
-The client IDs are limited to 16 bits because of the current implementation (see
-Doorbell register in 'PCI device registers' subsection). Hence only 65536
-clients are supported.
-
-All the file descriptors (fd to the shared memory, eventfds for each client)
-are passed to clients using SCM_RIGHTS over the server unix socket.
-
-Apart from the current ivshmem implementation in QEMU, an ivshmem client has
-been provided in qemu.git/contrib/ivshmem-client for debug.
-
-*QEMU as an ivshmem client*
-
-At initialisation, when creating the ivshmem device, QEMU first receives a
-protocol version and closes communication with server if it does not match.
-Then, QEMU gets its ID from the server then makes it available through BAR0
-IVPosition register for the VM to use (see 'PCI device registers' subsection).
-QEMU then uses the fd to the shared memory to map it to BAR2.
-eventfds for all other clients received from the server are stored to implement
-BAR0 Doorbell register (see 'PCI device registers' subsection).
-Finally, eventfds assigned to this QEMU process are used to send interrupts in
-this VM.
-
-*PCI device registers*
-
-From the VM point of view, the ivshmem PCI device supports 4 registers of
-32-bits each.
-
-enum ivshmem_registers {
-    IntrMask = 0,
-    IntrStatus = 4,
-    IVPosition = 8,
-    Doorbell = 12
-};
-
-The first two registers are the interrupt mask and status registers.  Mask and
-status are only used with pin-based interrupts.  They are unused with MSI
-interrupts.
-
-Status Register: The status register is set to 1 when an interrupt occurs.
-
-Mask Register: The mask register is bitwise ANDed with the interrupt status
-and the result will raise an interrupt if it is non-zero.  However, since 1 is
-the only value the status will be set to, it is only the first bit of the mask
-that has any effect.  Therefore interrupts can be masked by setting the first
-bit to 0 and unmasked by setting the first bit to 1.
-
-IVPosition Register: The IVPosition register is read-only and reports the
-guest's ID number.  The guest IDs are non-negative integers.  When using the
-server, since the server is a separate process, the VM ID will only be set when
-the device is ready (shared memory is received from the server and accessible
-via the device).  If the device is not ready, the IVPosition will return -1.
-Applications should ensure that they have a valid VM ID before accessing the
-shared memory.
-
-Doorbell Register:  To interrupt another guest, a guest must write to the
-Doorbell register.  The doorbell register is 32-bits, logically divided into
-two 16-bit fields.  The high 16-bits are the guest ID to interrupt and the low
-16-bits are the interrupt vector to trigger.  The semantics of the value
-written to the doorbell depends on whether the device is using MSI or a regular
-pin-based interrupt.  In short, MSI uses vectors while regular interrupts set
-the status register.
-
-Regular Interrupts
-
-If regular interrupts are used (due to either a guest not supporting MSI or the
-user specifying not to use them on startup) then the value written to the lower
-16-bits of the Doorbell register results is arbitrary and will trigger an
-interrupt in the destination guest.
-
-Message Signalled Interrupts
-
-An ivshmem device may support multiple MSI vectors.  If so, the lower 16-bits
-written to the Doorbell register must be between 0 and the maximum number of
-vectors the guest supports.  The lower 16 bits written to the doorbell is the
-MSI vector that will be raised in the destination guest.  The number of MSI
-vectors is configurable but it is set when the VM is started.
-
-The important thing to remember with MSI is that it is only a signal, no status
-is set (since MSI interrupts are not shared).  All information other than the
-interrupt itself should be communicated via the shared memory region.  Devices
-supporting multiple MSI vectors can use different vectors to indicate different
-events have occurred.  The semantics of interrupt vectors are left to the
-user's discretion.