1Inter-VM Shared Memory device 2----------------------------- 3 4On Linux hosts, a shared memory device is available. The basic syntax 5is: 6 7.. parsed-literal:: 8 9 |qemu_system_x86| -device ivshmem-plain,memdev=hostmem 10 11where hostmem names a host memory backend. For a POSIX shared memory 12backend, use something like 13 14:: 15 16 -object memory-backend-file,size=1M,share,mem-path=/dev/shm/ivshmem,id=hostmem 17 18If desired, interrupts can be sent between guest VMs accessing the same 19shared memory region. Interrupt support requires using a shared memory 20server and using a chardev socket to connect to it. The code for the 21shared memory server is qemu.git/contrib/ivshmem-server. An example 22syntax when using the shared memory server is: 23 24.. parsed-literal:: 25 26 # First start the ivshmem server once and for all 27 ivshmem-server -p pidfile -S path -m shm-name -l shm-size -n vectors 28 29 # Then start your qemu instances with matching arguments 30 |qemu_system_x86| -device ivshmem-doorbell,vectors=vectors,chardev=id 31 -chardev socket,path=path,id=id 32 33When using the server, the guest will be assigned a VM ID (>=0) that 34allows guests using the same server to communicate via interrupts. 35Guests can read their VM ID from a device register (see 36ivshmem-spec.txt). 37 38Migration with ivshmem 39~~~~~~~~~~~~~~~~~~~~~~ 40 41With device property ``master=on``, the guest will copy the shared 42memory on migration to the destination host. With ``master=off``, the 43guest will not be able to migrate with the device attached. In the 44latter case, the device should be detached and then reattached after 45migration using the PCI hotplug support. 46 47At most one of the devices sharing the same memory can be master. The 48master must complete migration before you plug back the other devices. 49 50ivshmem and hugepages 51~~~~~~~~~~~~~~~~~~~~~ 52 53Instead of specifying the <shm size> using POSIX shm, you may specify a 54memory backend that has hugepage support: 55 56.. parsed-literal:: 57 58 |qemu_system_x86| -object memory-backend-file,size=1G,mem-path=/dev/hugepages/my-shmem-file,share,id=mb1 59 -device ivshmem-plain,memdev=mb1 60 61ivshmem-server also supports hugepages mount points with the ``-m`` 62memory path argument. 63