Upcall is a direct communication tool between VMM and guest developed upon vsock. The server side of the upcall is a driver in guest kernel (kernel patches are needed for this feature) and it'll start to serve the requests after the kernel starts. And the client side is in Dragonball VMM , it'll be a thread that communicates with vsock through uds.
We want to keep the lightweight of the VM through the implementation of the upcall.
We define specific operations in the device manager service (one of the services in upcall we developed) to perform device hotplug / hot-unplug including vCPU hotplug, virtio-mmio hotplug, and memory hotplug. We have accomplished device hotplug / hot-unplug directly through upcall in order to avoid the virtualization of ACPI to minimize virtual machines overhead. And there could be many other uses if other services are implemented.
Upcall needs a server in the guest kernel which will be several kernel patches for the upcall server itself and different services registered in the upcall server. It's currently tested on upstream Linux kernel 5.10.
To make it easy for users to use, we have open-source the upcall guest patches in Dragonball experimental guest patches and develop upcall support in Kata guest kernel building script.
You could use following command to download the upstream kernel (currently Dragonball uses 5.10.25) and put the upcall patches and other Kata patches into kernel code.
sh build-kernel.sh -e -t dragonball -f setup
-e here means experimental, mainly because upcall patches are not in upstream Linux kernel.
-t dragonball is for specifying hypervisor type
-f is for generating .config file
After this command, the kernel code with upcall and related .config file are all set up in the directory kata-linux-dragonball-experimental-5.10.25-[config version]. You can either manually compile the kernel with make command or following Document for build-kernel.sh to build and use this guest kernel.
Also, a client-side is also needed in VMM. Dragonball has already open-source the way to implement upcall client and Dragonball compiled with dbs-upcall feature will enable Dragonball client side.