[Qemu-devel] vhost-net - virtio - multiqueue - rx queue selection

[Qemu-devel] vhost-net - virtio - multiqueue - rx queue selection

[Sujith Sankar]

Hi folks,

I have a question regarding the algorithm used to select rx queue of
the VM in the case of multiqueue virtio-net on KVM.

Given a 4-tuple, how can we calculate the index of the receive queue
that virtio-net is going pick for that flow?  From some of the email
threads in some of the relevant forums, it seems RSS is not used.  But
documentations/threads do not talk about how a queue is picked.

Could you please help me with this?

Thanks,
-Sujith

Re: [Qemu-devel] vhost-net - virtio - multiqueue - rx queue selection

[Stefan Hajnoczi]

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On Thu, Apr 20, 2017 at 10:07:33AM +0530, Sujith Sankar wrote:
> I have a question regarding the algorithm used to select rx queue of
> the VM in the case of multiqueue virtio-net on KVM.
> 
> Given a 4-tuple, how can we calculate the index of the receive queue
> that virtio-net is going pick for that flow?  From some of the email
> threads in some of the relevant forums, it seems RSS is not used.  But
> documentations/threads do not talk about how a queue is picked.
> 
> Could you please help me with this?

CCing Jason Wang, QEMU net maintainer.

Stefan

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Re: [Qemu-devel] vhost-net - virtio - multiqueue - rx queue selection

[Jason Wang]

On 2017年05月02日 21:07, Stefan Hajnoczi wrote:
> On Thu, Apr 20, 2017 at 10:07:33AM +0530, Sujith Sankar wrote:
>> I have a question regarding the algorithm used to select rx queue of
>> the VM in the case of multiqueue virtio-net on KVM.
>>
>> Given a 4-tuple, how can we calculate the index of the receive queue
>> that virtio-net is going pick for that flow?  From some of the email
>> threads in some of the relevant forums, it seems RSS is not used.  But
>> documentations/threads do not talk about how a queue is picked.
>>
>> Could you please help me with this?
> CCing Jason Wang, QEMU net maintainer.
>
> Stefan

Automatic flow steering is mandatory in the spec (see 5.1.6.5.5):

"
When multiqueue is enabled, the device MUST use automatic receive 
steering based on packet flow. Programming of the receive steering 
classificator is implicit. After the driver transmitted a packet of a 
flow on transmitqX, the device SHOULD cause incoming packets for that 
flow to be steered to receiveqX. For uni-directional protocols, or where 
no packets have been transmitted yet, the device MAY steer a packet to a 
random queue out of the specified receiveq1…receiveqn.
"

We plan to add guest visible filters (e.g RSS) support, and may even 
consider offload it to hardware.

Thanks