[Qemu-devel] vhost-pci and virtio-vhost-user

[Stefan Hajnoczi <stefanha@redhat.com>]

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Hi Wei,
I wanted to summarize the differences between the vhost-pci and
virtio-vhost-user approaches because previous discussions may have been
confusing.

vhost-pci defines a new virtio device type for each vhost device type
(net, scsi, blk).  It therefore requires a virtio device driver for each
device type inside the slave VM.

Adding a new device type requires:
1. Defining a new virtio device type in the VIRTIO specification.
3. Implementing a new QEMU device model.
2. Implementing a new virtio driver.

virtio-vhost-user is a single virtio device that acts as a vhost-user
protocol transport for any vhost device type.  It requires one virtio
driver inside the slave VM and device types are implemented using
existing vhost-user slave libraries (librte_vhost in DPDK and
libvhost-user in QEMU).

Adding a new device type to virtio-vhost-user involves:
1. Adding any new vhost-user protocol messages to the QEMU
   virtio-vhost-user device model.
2. Adding any new vhost-user protocol messages to the vhost-user slave
   library.
3. Implementing the new device slave.

The simplest case is when no new vhost-user protocol messages are
required for the new device.  Then all that's needed for
virtio-vhost-user is a device slave implementation (#3).  That slave
implementation will also work with AF_UNIX because the vhost-user slave
library hides the transport (AF_UNIX vs virtio-vhost-user).  Even
better, if another person has already implemented that device slave to
use with AF_UNIX then no new code is needed for virtio-vhost-user
support at all!

If you compare this to vhost-pci, it would be necessary to design a new
virtio device, implement it in QEMU, and implement the virtio driver.
Much of virtio driver is more or less the same thing as the vhost-user
device slave but it cannot be reused because the vhost-user protocol
isn't being used by the virtio device.  The result is a lot of
duplication in DPDK and other codebases that implement vhost-user
slaves.

The way that vhost-pci is designed means that anyone wishing to support
a new device type has to become a virtio device designer.  They need to
map vhost-user protocol concepts to a new virtio device type.  This will
be time-consuming for everyone involved (e.g. the developer, the VIRTIO
community, etc).

The virtio-vhost-user approach stays at the vhost-user protocol level as
much as possible.  This way there are fewer concepts that need to be
mapped by people adding new device types.  As a result, it will allow
virtio-vhost-user to keep up with AF_UNIX vhost-user and grow because
it's easier to work with.

What do you think?

Stefan

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