Inter-VM Shared Memory
Overview
As Zephyr is enabled to run as a guest OS on Qemu and ACRN it might be necessary to make VMs aware of each other, or aware of the host. This is made possible by exposing a shared memory among parties via a feature called ivshmem, which stands for inter-VM Shared Memory.
The two types are supported: a plain shared memory (ivshmem-plain) or a shared memory with the ability for a VM to generate an interruption on another, and thus to be interrupted as well itself (ivshmem-doorbell).
Please refer to the official Qemu ivshmem documentation for more information.
Support
Zephyr supports both versions: plain and doorbell. Ivshmem driver can be built
by enabling CONFIG_IVSHMEM. By default, this will expose the plain
version. CONFIG_IVSHMEM_DOORBELL needs to be enabled to get the
doorbell version.
Because the doorbell version uses MSI-X vectors to support notification vectors,
the CONFIG_IVSHMEM_MSI_X_VECTORS has to be tweaked to the number of
vectors that will be needed.
Note that a tiny shell module can be exposed to test the ivshmem feature by
enabling CONFIG_IVSHMEM_SHELL.
ivshmem-v2
Zephyr also supports ivshmem-v2:
https://github.com/siemens/jailhouse/blob/master/Documentation/ivshmem-v2-specification.md
This is primarily used for IPC in the Jailhouse hypervisor (e.g. Inter-VM Shared Memory (ivshmem) Ethernet). It is also possible to use ivshmem-v2 without Jailhouse by building the Siemens fork of QEMU, and modifying the QEMU launch flags: