netdev - Re: [net-next RFC PATCH 5/5] virtio-net: flow director support

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Date:	Wed, 7 Dec 2011 15:04:56 +0000
From:	Stefan Hajnoczi <stefanha@...il.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	krkumar2@...ibm.com, kvm@...r.kernel.org, mst@...hat.com,
	netdev@...r.kernel.org, rusty@...tcorp.com.au,
	virtualization@...ts.linux-foundation.org, levinsasha928@...il.com,
	bhutchings@...arflare.com
Subject: Re: [net-next RFC PATCH 5/5] virtio-net: flow director support

On Wed, Dec 7, 2011 at 12:10 PM, Jason Wang <jasowang@...hat.com> wrote:
> On 12/07/2011 05:08 PM, Stefan Hajnoczi wrote:
> [...]
>
>>> >  Consider the complexity of the host nic each with their own steering
>>> >  features,  this series make the first step with minimal effort to try
>>> > to let
>>> >  guest driver and host tap/macvtap co-operate like what physical nic
>>> > does.
>>> >  There may be other method, but performance numbers is also needed to
>>> > give
>>> >  the answer.
>>
>> I agree that performance results for this need to be shown.
>>
>> My original point is really that it's not a good idea to take
>> individual steps without a good big picture because this will change
>> the virtio-net device specification.  If this turns out to be a dead
>> end then hosts will need to continue to support the interface forever
>> (legacy guests could still try to use it).  So please first explain
>> what the full stack picture is going to look like and how you think it
>> will lead to better performance.  You don't need to have all the code
>> or evidence, but just enough explanation so we see where this is all
>> going.
>
> I think I mention too little in the cover message.
>
> There's no much changes with Krishna's series except the method that
> choosing a rx virtqueue. Since original series use different hash methods in
> host (rxhash) and guest (txhash), a different virtqueue were chose for a
> flow which could lead packets of a flow to be handled by different vhost
> thread and vcpu. This may damage the performance.
>
> This series tries to let one vhost thread to process the packets of a flow
> and also let the packets to be sent directly to a vcpu local to the thread
> process the data. This is done by letting guest tell the desired queue form
> which it want to receive the pakcet of a dedicated flow.
>
> So passing the hash from host to guest is needed to get the same hash in the
> two sides. Then a guest programmable hash to queue table were introduced and
> guest co-operate with the host through accelerate RFS in guest.

Thanks for the context, that helps me understand it a little better.

Stefan
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