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Date:   Tue, 22 Nov 2016 16:59:31 +0200
From:   "Michael S. Tsirkin" <>
To:     John Fastabend <>
Subject: Re: [net-next PATCH v2 4/5] virtio_net: add dedicated XDP transmit

On Tue, Nov 22, 2016 at 12:17:40AM -0800, John Fastabend wrote:
> On 16-11-21 03:13 PM, Michael S. Tsirkin wrote:
> > On Sat, Nov 19, 2016 at 06:51:04PM -0800, John Fastabend wrote:
> >> XDP requires using isolated transmit queues to avoid interference
> >> with normal networking stack (BQL, NETDEV_TX_BUSY, etc). This patch
> >> adds a XDP queue per cpu when a XDP program is loaded and does not
> >> expose the queues to the OS via the normal API call to
> >> netif_set_real_num_tx_queues(). This way the stack will never push
> >> an skb to these queues.
> >>
> >> However virtio/vhost/qemu implementation only allows for creating
> >> TX/RX queue pairs at this time so creating only TX queues was not
> >> possible. And because the associated RX queues are being created I
> >> went ahead and exposed these to the stack and let the backend use
> >> them. This creates more RX queues visible to the network stack than
> >> TX queues which is worth mentioning but does not cause any issues as
> >> far as I can tell.
> >>
> >> Signed-off-by: John Fastabend <>
> > 
> > FYI what's supposed to happen is packets from the same
> > flow going in the reverse direction will go on the
> > same queue.
> > 
> > This might come in handy when implementing RX XDP.
> > 
> Yeah but if its the first packet not part of a flow then presumably it
> can pick any queue but its worth keeping in mind certainly.
> .John

Oh I agree, absolutely. This was just a FYI in case it comes useful
as an optimization down the road.


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