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22 > and guests should have a straightforward, efficient, standard and extensible
26 Virtio devices use virtqueues to transport data efficiently. Virtqueue is a set
29 where the QEMU itself exposes a virtual PCI device and the guest OS communicates
30 with it using a specific Virtio PCI driver. With only Virtio involved, it's
33 Vhost is a protocol for devices accessible via inter-process communication.
35 directly to Virtio devices. This allows a Vhost device, exposed by an SPDK
36 application, to be accessed directly by a guest OS inside a QEMU process with
41 The initial vhost implementation is a part of the Linux kernel and uses ioctl
43 SPDK to expose a vhost device is Vhost-user protocol.
50 > plane needed to establish virtqueue sharing with a user space process on the
51 > same host. It uses communication over a Unix domain socket to share file
59 > the external process consuming the virtio queues, for example a software
60 > Ethernet switch running in user space, such as Snabbswitch, or a block
61 > device back-end processing read and write to a virtual disk.
63 > The front-end and back-end can be either a client (i.e. connecting) or
66 SPDK vhost is a Vhost-user back-end server. It exposes Unix domain sockets and
81 the front-end and the back-end expose a list of their implemented features and
82 upon negotiation they choose a common set of those. Most of these features are
87 physically-discontiguous regions and Vhost-user specification puts a limit on
88 their number - currently 8. The driver sends a single message for each region with
95   is a physical address inside the guest)
102 Drivers may also request a device config, consisting of e.g. disk geometry.
115 If multiqueue feature has been negotiated, the driver has to send a specific
134 And a Virtio-SCSI request as follows.
146 to be converted into a chain of such descriptors. A single descriptor can be
157 /* This marks a buffer as continuing via the next field. */
159 /* This marks a buffer as device write-only (otherwise device read-only). */
175 for polling virtqueues. For each descriptor, the device performs a lookup in
176 the Vhost-user memory region table and goes through a gpa_to_vva translation
178 and response data to be contained within a single memory region. I/O buffers
193 be discussed in this document. For the highest performance, a poll-mode @ref virtio