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+.. _pcsys_005fivshmem:
+
+Inter-VM Shared Memory device
+-----------------------------
+
+On Linux hosts, a shared memory device is available. The basic syntax
+is:
+
+.. parsed-literal::
+
+   |qemu_system_x86| -device ivshmem-plain,memdev=hostmem
+
+where hostmem names a host memory backend. For a POSIX shared memory
+backend, use something like
+
+::
+
+   -object memory-backend-file,size=1M,share,mem-path=/dev/shm/ivshmem,id=hostmem
+
+If desired, interrupts can be sent between guest VMs accessing the same
+shared memory region. Interrupt support requires using a shared memory
+server and using a chardev socket to connect to it. The code for the
+shared memory server is qemu.git/contrib/ivshmem-server. An example
+syntax when using the shared memory server is:
+
+.. parsed-literal::
+
+   # First start the ivshmem server once and for all
+   ivshmem-server -p pidfile -S path -m shm-name -l shm-size -n vectors
+
+   # Then start your qemu instances with matching arguments
+   |qemu_system_x86| -device ivshmem-doorbell,vectors=vectors,chardev=id
+                    -chardev socket,path=path,id=id
+
+When using the server, the guest will be assigned a VM ID (>=0) that
+allows guests using the same server to communicate via interrupts.
+Guests can read their VM ID from a device register (see
+ivshmem-spec.txt).
+
+Migration with ivshmem
+~~~~~~~~~~~~~~~~~~~~~~
+
+With device property ``master=on``, the guest will copy the shared
+memory on migration to the destination host. With ``master=off``, the
+guest will not be able to migrate with the device attached. In the
+latter case, the device should be detached and then reattached after
+migration using the PCI hotplug support.
+
+At most one of the devices sharing the same memory can be master. The
+master must complete migration before you plug back the other devices.
+
+ivshmem and hugepages
+~~~~~~~~~~~~~~~~~~~~~
+
+Instead of specifying the <shm size> using POSIX shm, you may specify a
+memory backend that has hugepage support:
+
+.. parsed-literal::
+
+   |qemu_system_x86| -object memory-backend-file,size=1G,mem-path=/dev/hugepages/my-shmem-file,share,id=mb1
+                    -device ivshmem-plain,memdev=mb1
+
+ivshmem-server also supports hugepages mount points with the ``-m``
+memory path argument.