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

[Jason Wang]

On 2018年01月15日 18:43, Wei Wang wrote:
> On 01/15/2018 04:34 PM, Jason Wang wrote:
> > On 2018年01月15日 15:59, Wei Wang wrote:
> > > On 01/15/2018 02:56 PM, Jason Wang wrote:
> > > > On 2018年01月12日 18:18, Stefan Hajnoczi wrote:
> > > > >
> > > >
> > > > I just fail understand why we can't do software defined network or 
> > > > storage with exist virtio device/drivers (or are there any 
> > > > shortcomings that force us to invent new infrastructure).
> > >
> > > Existing virtio-net works with a host central vSwitch, and it has 
> > > the following disadvantages:
> > > 1) long code/data path;
> > > 2) poor scalability; and
> > > 3) host CPU sacrifice
> >
> > Please show me the numbers.
>
> Sure. For 64B packet transmission between two VMs: vhost-user reports 
> ~6.8Mpps, and vhost-pci reports ~11Mpps, which is ~1.62x faster.

This result is kind of incomplete. So still many questions left:

- What's the configuration of the vhost-user?
- What's the result of e.g 1500 byte?
- You said it improves scalability, at least I can't get this conclusion 
just from what you provide here
- You suspect long code/data path, but no latency numbers to prove it

> > > Vhost-pci solves the above issues by providing a point-to-point 
> > > communication between VMs. No matter how the control path would look 
> > > like finally, the key point is that the data path is P2P between VMs.
> > >
> > > Best,
> > > Wei
> >
> > Well, I think I've pointed out several times in the replies of 
> > previous versions. Both vhost-pci-net and virtio-net is an ethernet 
> > device, which is not tied to a central vswitch for sure. There're 
> > just too many methods or tricks which can be used to build a point to 
> > point data path.
>
>
> Could you please show an existing example that makes virtio-net work 
> without a host vswitch/bridge?

For vhost-user, it's as simple as a testpmd which does io forwarding 
between two vhost ports? For kernel, you can do even more tricks, tc, 
bpf or whatever others.

> Could you also share other p2p data path solutions that you have in 
> mind? Thanks.
>
>
> Best,
> Wei

So my point stands still: both vhost-pci-net and virtio-net are ethernet 
devices, any ethernet device can connect to each other directly without 
switch. Saying virtio-net can not connect to each other directly without 
a switch obviously make no sense, it's a network topology issue for 
sure. Even if it was not a typical setup or configuration, extending the 
exist backends is 1st choice unless you can prove there're any design 
limitations of exist solutions.

Thanks