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

[Jason Wang <jasowang@redhat.com>]

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