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Inter-VM shared memory device

The Inter-VM shared memory device (ivshmem) is designed to share a memory region between a guest and the host. In order for all guests to be able to pick up the shared memory area, it is modeled as a PCI device exposing said memory to the guest as a PCI BAR.

Device Specification is available at https://www.qemu.org/docs/master/specs/ivshmem-spec.html.

Now we support setting a backend file to share data between host and guest. In other words, we only support ivshmem-plain and ivshmem-doorbell is not supported yet.

Usage

--ivshmem, an optional argument, can be passed to enable ivshmem device. This argument takes a file as a path value and a file size as a size value. The size value must be 2^n.

--ivshmem <ivshmem>  device backend file "path=</path/to/a/file>,size=<file_size>"

Example

Create a file with a size bigger than passed to cloud-hypervisor:

truncate -s 1M /tmp/ivshmem.data

Start application to mmap the file data to a memory region:

./cloud-hypervisor \
    --api-socket /tmp/cloud-hypervisor.sock \
    --kernel vmlinux \
    --disk path=focal-server-cloudimg-amd64.raw \
    --cpus boot=4 \
    --memory size=1024M \
    --ivshmem path=/tmp/ivshmem.data,size=1M

Insmod an ivshmem device driver to enable the device. The file data will be mmapped to the PCI bar2 of ivshmem device, guest can r/w data by accessing this memory.

A simple example of ivshmem driver can be obtained from: https://github.com/lisongqian/clh-linux/commits/ch-6.12.8-ivshmem

The host process can r/w this data by remapping the /tmp/ivshmem.data.