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

[Jason Wang <jasowang@redhat.com>]

On 2018年01月11日 00:14, Stefan Hajnoczi wrote:
> 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

So a question is what's the motivation here?

Form what I'm understanding, vhost-pci tries to build a scalable V2V 
private datapath. But according to what you describe here, 
virito-vhost-user tries to make it possible to implement the device 
inside another VM. I understand the goal of vhost-pci could be done on 
top, but it looks to me it would then rather similar to the design of 
Xen driver domain. So I can not figure out how it can be done in a high 
performance way.

Thanks