netdev - Re: [net-next PATCH v2 4/5] virtio_net: add dedicated XDP transmit queues

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Date:   Tue, 22 Nov 2016 00:17:40 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     daniel@...earbox.net, eric.dumazet@...il.com, kubakici@...pl,
        shm@...ulusnetworks.com, davem@...emloft.net,
        alexei.starovoitov@...il.com, netdev@...r.kernel.org,
        bblanco@...mgrid.com, john.r.fastabend@...el.com,
        brouer@...hat.com, tgraf@...g.ch
Subject: Re: [net-next PATCH v2 4/5] virtio_net: add dedicated XDP transmit
 queues

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 <john.r.fastabend@...el.com>
> 
> 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