netdev - Re: [PATCHv4] virtio-spec: virtio network device multiqueue support

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Message-ID: <87har3dc4o.fsf@rustcorp.com.au>
Date:	Wed, 12 Sep 2012 15:19:11 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Jason Wang <jasowang@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	netdev@...r.kernel.org, pbonzini@...hat.com,
	levinsasha928@...il.com, rick.jones2@...com,
	Tom Herbert <therbert@...gle.com>
Subject: Re: [PATCHv4] virtio-spec: virtio network device multiqueue support

Jason Wang <jasowang@...hat.com> writes:
> On 09/10/2012 02:33 PM, Michael S. Tsirkin wrote:
>> A final addition: what you suggest above would be
>> "TX follows RX", right?

BTW, yes.  But it's a weird way to express what the nic is doing.

>> It is in anticipation of something like that, that I made
>> steering programming so generic.

>> I think TX follows RX is more immediately useful for reasons above
>> but we can add both to spec and let drivers and devices
>> decide what they want to support.

You mean "RX follows TX"?  ie. accelerated RFS.  I agree.

Perhaps Tom can explain how we avoid out-of-order receive for the
accelerated RFS case?  It's not clear to me, but we need to be able to
do that for virtio-net if it implements accelerated RFS.

> AFAIK, ixgbe does "rx follows tx". The only differences between ixgbe 
> and virtio-net is that ixgbe driver programs the flow director during 
> packet transmission but we suggest to do it silently in the device for 
> simplicity.

Implying the receive queue by xmit will be slightly laggy.  Don't know
if that's a problem.

Cheers,
Rusty.
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