# Inter-VM Shared Memory Flat Device

The ivshmem-flat device is meant to be used on machines that lack a PCI bus, making them unsuitable for the use of the traditional ivshmem device modeled as a PCI device. Machines like those with a Cortex-M MCU are good candidates to use the ivshmem-flat device. Also, since the flat version maps the control and status registers directly to the memory, it requires a quite tiny “device driver” to interact with other VMs, which is useful in some RTOSes, like Zephyr, which usually run on constrained resource targets.

Similar to the ivshmem device, the ivshmem-flat device supports both peer notification via HW interrupts and Inter-VM shared memory. This allows the device to be used together with the traditional ivshmem, enabling communication between, for instance, an aarch64 VM (using the traditional ivshmem device and running Linux), and an arm VM (using the ivshmem-flat device and running Zephyr instead).

The ivshmem-flat device does not support the use of a `memdev` option (see ivshmem.rst for more details). It relies on the ivshmem server to create and distribute the proper shared memory file descriptor and the eventfd(s) to notify (interrupt) the peers. Therefore, to use this device, it is always necessary to have an ivshmem server up and running for proper device creation.

Although the ivshmem-flat supports both peer notification (interrupts) and shared memory, the interrupt mechanism is optional. If no input IRQ is specified for the device it is disabled, preventing the VM from notifying or being notified by other VMs (a warning will be displayed to the user to inform the IRQ mechanism is disabled). The shared memory region is always present.

The MMRs (INTRMASK, INTRSTATUS, IVPOSITION, and DOORBELL registers) offsets at the MMR region, and their functions, follow the ivshmem spec, so they work exactly as in the ivshmem PCI device (see ./specs/ivshmem-spec.txt).