virtio9p

Virtio9p, also known as virtio-9p, is a virtio transport device that implements the 9P (Plan 9) filesystem protocol over the virtio virtualization interface. It enables sharing of files and directories between a host system and a guest virtual machine by exporting a host directory to the guest through a paravirtualized 9P filesystem. The device is part of the Virtio family and is commonly used with QEMU/KVM.

In a typical setup, the host directory is exposed to the guest via QEMU's fsdev backend and a virtio-9p device, identified by a mount_tag. Inside the guest, the 9P filesystem can be mounted with the standard 9P mount tooling, using the virtio transport (trans=virtio) and a protocol version such as 9P2000.L. The result is a coherent, file-system-like view of the host directory inside the guest.

Implementation details: The Linux kernel provides a 9P client that can operate over the virtio transport; the host side uses a corresponding fsdev backend to export the path. The setup typically requires a kernel with virtio-9p support and a QEMU version that implements the virtio-9p device. Security and performance options include mount-time and I/O caching settings, as well as security_model parameters on the host side (for example, security_model=none in test environments).

Usage considerations: virtio9p is convenient for sharing source trees or development data between host and guest, but may not match the performance of native filesystems or dedicated shared folders for heavy I/O workloads. It is one of several virtio-based sharing methods, complementary to newer approaches such as virtiofs. See also 9P protocol, Virtio, QEMU, virtiofs.