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+
+Device Specification for Inter-VM shared memory device
+------------------------------------------------------
+
+The Inter-VM shared memory device is designed to share a region of memory to
+userspace in multiple virtual guests.  The memory region does not belong to any
+guest, but is a POSIX memory object on the host.  Optionally, the device may
+support sending interrupts to other guests sharing the same memory region.
+
+
+The Inter-VM PCI device
+-----------------------
+
+*BARs*
+
+The device supports three BARs.  BAR0 is a 1 Kbyte MMIO region to support
+registers.  BAR1 is used for MSI-X when it is enabled in the device.  BAR2 is
+used to map the shared memory object from the host.  The size of BAR2 is
+specified when the guest is started and must be a power of 2 in size.
+
+*Registers*
+
+The device currently supports 4 registers of 32-bits each.  Registers
+are used for synchronization between guests sharing the same memory object when
+interrupts are supported (this requires using the shared memory server).
+
+The server assigns each VM an ID number and sends this ID number to the Qemu
+process when the guest starts.
+
+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
+
+A 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.
+
+
+Usage in the Guest
+------------------
+
+The shared memory device is intended to be used with the provided UIO driver.
+Very little configuration is needed.  The guest should map BAR0 to access the
+registers (an array of 32-bit ints allows simple writing) and map BAR2 to
+access the shared memory region itself.  The size of the shared memory region
+is specified when the guest (or shared memory server) is started.  A guest may
+map the whole shared memory region or only part of it.