# Inter-VM Shared Memory device

On Linux hosts, a shared memory device is available. The basic syntax is:

    qemu-system-x86_64 -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:

    # 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_64 -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:

    qemu-system-x86_64 -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.