1.. _pcsys_005fivshmem: 2 3Inter-VM Shared Memory device 4----------------------------- 5 6On Linux hosts, a shared memory device is available. The basic syntax 7is: 8 9.. parsed-literal:: 10 11 |qemu_system_x86| -device ivshmem-plain,memdev=hostmem 12 13where hostmem names a host memory backend. For a POSIX shared memory 14backend, use something like 15 16:: 17 18 -object memory-backend-file,size=1M,share,mem-path=/dev/shm/ivshmem,id=hostmem 19 20If desired, interrupts can be sent between guest VMs accessing the same 21shared memory region. Interrupt support requires using a shared memory 22server and using a chardev socket to connect to it. The code for the 23shared memory server is qemu.git/contrib/ivshmem-server. An example 24syntax when using the shared memory server is: 25 26.. parsed-literal:: 27 28 # First start the ivshmem server once and for all 29 ivshmem-server -p pidfile -S path -m shm-name -l shm-size -n vectors 30 31 # Then start your qemu instances with matching arguments 32 |qemu_system_x86| -device ivshmem-doorbell,vectors=vectors,chardev=id 33 -chardev socket,path=path,id=id 34 35When using the server, the guest will be assigned a VM ID (>=0) that 36allows guests using the same server to communicate via interrupts. 37Guests can read their VM ID from a device register (see 38ivshmem-spec.txt). 39 40Migration with ivshmem 41~~~~~~~~~~~~~~~~~~~~~~ 42 43With device property ``master=on``, the guest will copy the shared 44memory on migration to the destination host. With ``master=off``, the 45guest will not be able to migrate with the device attached. In the 46latter case, the device should be detached and then reattached after 47migration using the PCI hotplug support. 48 49At most one of the devices sharing the same memory can be master. The 50master must complete migration before you plug back the other devices. 51 52ivshmem and hugepages 53~~~~~~~~~~~~~~~~~~~~~ 54 55Instead of specifying the <shm size> using POSIX shm, you may specify a 56memory backend that has hugepage support: 57 58.. parsed-literal:: 59 60 |qemu_system_x86| -object memory-backend-file,size=1G,mem-path=/dev/hugepages/my-shmem-file,share,id=mb1 61 -device ivshmem-plain,memdev=mb1 62 63ivshmem-server also supports hugepages mount points with the ``-m`` 64memory path argument. 65