vHost

A pure rust library for vDPA, vhost and vhost-user.

The vhost crate aims to help implementing dataplane for virtio backend drivers. It supports three different types of dataplane drivers:

• vhost: the dataplane is implemented by linux kernel
• vhost-user: the dataplane is implemented by dedicated vhost-user servers
• vDPA(vhost DataPath Accelerator): the dataplane is implemented by hardwares

The main relationship among Traits and Structs exported by the vhost crate is as below:

Kernel-based vHost Backend Drivers

The vhost drivers in Linux provide in-kernel virtio device emulation. Normally the hypervisor userspace process emulates I/O accesses from the guest. Vhost puts virtio emulation code into the kernel, taking hypervisor userspace out of the picture. This allows device emulation code to directly call into kernel subsystems instead of performing system calls from userspace. The hypervisor relies on ioctl based interfaces to control those in-kernel vhost drivers, such as vhost-net, vhost-scsi and vhost-vsock etc.

vHost-user Backend Drivers

The vhost-user protocol aims to implement vhost backend drivers in userspace, which complements the ioctl interface used to control the vhost implementation in the Linux kernel. It implements the control plane needed to establish virtqueue sharing with a user space process on the same host. It uses communication over a Unix domain socket to share file descriptors in the ancillary data of the message.

The protocol defines two sides of the communication, frontend and backend. Frontend is the application that shares its virtqueues, backend is the consumer of the virtqueues. Frontend and backend can be either a client (i.e. connecting) or server (listening) in the socket communication.

Postcopy support

To enabled POSTCOPY_* messages support there is a postcopy feature. Due to how Xen handles memory mappings the postcopy feature is not compatible with xen feature. Enabling both at the same time will result in a compilation error.

Xen support

Supporting Xen requires special handling while mapping the guest memory. The vm-memory crate implements xen memory mapping support via a separate feature xen, and this crate uses the same feature name to enable Xen support.

Also, for xen mappings, the memory regions passed by the frontend contains few extra fields as described in the vhost-user protocol documentation.

It was decided by the rust-vmm maintainers to keep the interface simple and build the crate for either standard Unix memory mapping or Xen, and not both.