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

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Message-ID: <87wqzqpz7p.fsf@rustcorp.com.au>
Date:	Wed, 19 Sep 2012 11:10:10 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Tom Herbert <therbert@...gle.com>
Cc:	Jason Wang <jasowang@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	pbonzini@...hat.com, levinsasha928@...il.com, rick.jones2@...com
Subject: Re: [PATCHv4] virtio-spec: virtio network device multiqueue support

Tom Herbert <therbert@...gle.com> writes:
> On Tue, Sep 11, 2012 at 10:49 PM, Rusty Russell <rusty@...tcorp.com.au>wrote:
>> 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 ooo RX is possible with accelerated RFS.  We have an algorithm that
> prevents this for RFS case by deferring a migration to a new queue as long
> as it's possible that a flow might have outstanding packets on the old
> queue.  I suppose this could be implemented in the device for the HW
> queues, but I don't think it would be easy to cover all cases where packets
> were already in transit to the host or other cases where host and device
> queues are out of sync.

Having gone to such great lengths to avoid ooo for RFS, I don't think
DaveM would be happy if we allow it for virtio_net.

So, how *would* we implement such a thing for a "hardware" device?  What
if the device will only change the receive queue if the old receive
queue is empty?

Cheers,
Rusty.



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