vm_tools/docs/vsock.md - third_party/platform2 - Git at Google

 # virtio-vsock

 virtio-vsock provides a way for applications running on a guest VM and the host
 system to communicate with each other using the standard socket interface
 (`socket`, `connect`, `bind`, `listen`, `accept`).  It defines a new socket
 address family (`AF_VSOCK`) and uses a (context id, port) pair of integers for
 identifying processes.  The host system always has 2 as its context id while
 each guest VM is assigned a unique context id on startup.

 ## vhost

 virtio-vsock is a vhost-based virtio device.  This means that the host kernel
 manages all the data transfer while the hypervisor is only responsible for
 dealing with control information.  This diagram gives a high-level overview:

 ![vhost](vhost.png)

 An `irqfd` is a mechanism for injecting a specific interrupt into the guest VM
 using an [eventfd](http://man7.org/linux/man-pages/man2/eventfd.2.html).  The
 hypervisor creates an eventfd and uses a KVM ioctl to associate it with a
 particular interrupt for the guest VM.  Writes to this eventfd then trigger that
 interrupt in the guest.

 An `ioeventfd` does the same thing but reversed.  The hypervisor creates an
 eventfd and associates it with a particular region of the guest VM's memory.
 Writes to this memory region by the guest VM will make the eventfd readable
 without blocking.

 `irqfd` and `ioeventfd` are used to implement I/O for virtualized devices that
 does not block the guest VM cpu from running.

 For vhost devices, the hypervisor performs some additional steps. It notifies
 the vhost kernel module about the `ioeventfd` and the `irqfd` for the device as
 well as the guest memory regions assigned for the virtual queues for that
 device.  The vhost kernel module then takes care of listening on the
 `ioeventfd`, processing the requests from the guest VM, and triggering an
 interrupt in the guest on completion of the request through the `irqfd`.

 ## vsock

 vsock provides a new backend for the guest VM's networking stack that talks to
 the vhost_vsock host kernel module.  The host kernel module in turn uses the
 host kernel's networking stack to transfer data from the application in the
 guest VM to an application on the host system.

 ![vsock](vsock.png)
	# virtio-vsock

	virtio-vsock provides a way for applications running on a guest VM and the host
	system to communicate with each other using the standard socket interface
	(`socket`, `connect`, `bind`, `listen`, `accept`). It defines a new socket
	address family (`AF_VSOCK`) and uses a (context id, port) pair of integers for
	identifying processes. The host system always has 2 as its context id while
	each guest VM is assigned a unique context id on startup.

	## vhost

	virtio-vsock is a vhost-based virtio device. This means that the host kernel
	manages all the data transfer while the hypervisor is only responsible for
	dealing with control information. This diagram gives a high-level overview:

	![vhost](vhost.png)

	An `irqfd` is a mechanism for injecting a specific interrupt into the guest VM
	using an [eventfd](http://man7.org/linux/man-pages/man2/eventfd.2.html). The
	hypervisor creates an eventfd and uses a KVM ioctl to associate it with a
	particular interrupt for the guest VM. Writes to this eventfd then trigger that
	interrupt in the guest.

	An `ioeventfd` does the same thing but reversed. The hypervisor creates an
	eventfd and associates it with a particular region of the guest VM's memory.
	Writes to this memory region by the guest VM will make the eventfd readable
	without blocking.

	`irqfd` and `ioeventfd` are used to implement I/O for virtualized devices that
	does not block the guest VM cpu from running.

	For vhost devices, the hypervisor performs some additional steps. It notifies
	the vhost kernel module about the `ioeventfd` and the `irqfd` for the device as
	well as the guest memory regions assigned for the virtual queues for that
	device. The vhost kernel module then takes care of listening on the
	`ioeventfd`, processing the requests from the guest VM, and triggering an
	interrupt in the guest on completion of the request through the `irqfd`.

	## vsock

	vsock provides a new backend for the guest VM's networking stack that talks to
	the vhost_vsock host kernel module. The host kernel module in turn uses the
	host kernel's networking stack to transfer data from the application in the
	guest VM to an application on the host system.

	![vsock](vsock.png)