Subject: Re: [net-next PATCH v5 4/6] virtio_net: add dedicated XDP transmit queues

From: John Fastabend <[email-masked]>
Date: Thu, 8 Dec 2016 09:10:28 -0800

On 16-12-07 09:59 PM, Michael S. Tsirkin wrote:
> On Wed, Dec 07, 2016 at 12:12:23PM -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 <[email-masked]>
>> ---
>>  drivers/net/virtio_net.c |   30 ++++++++++++++++++++++++++++--
>>  1 file changed, 28 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index a009299..28b1196 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -114,6 +114,9 @@ struct virtnet_info {
>>  	/* # of queue pairs currently used by the driver */
>>  	u16 curr_queue_pairs;
>>  
>> +	/* # of XDP queue pairs currently used by the driver */
>> +	u16 xdp_queue_pairs;
>> +
>>  	/* I like... big packets and I cannot lie! */
>>  	bool big_packets;
>>  
>> @@ -1547,7 +1550,8 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog)
>>  	unsigned long int max_sz = PAGE_SIZE - sizeof(struct padded_vnet_hdr);
>>  	struct virtnet_info *vi = netdev_priv(dev);
>>  	struct bpf_prog *old_prog;
>> -	int i;
>> +	u16 xdp_qp = 0, curr_qp;
>> +	int i, err;
>>  
>>  	if ((dev->features & NETIF_F_LRO) && prog) {
>>  		netdev_warn(dev, "can't set XDP while LRO is on, disable LRO first\n");
>> @@ -1564,12 +1568,34 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog)
>>  		return -EINVAL;
>>  	}
>>  
>> +	curr_qp = vi->curr_queue_pairs - vi->xdp_queue_pairs;
>> +	if (prog)
>> +		xdp_qp = nr_cpu_ids;
>> +
>> +	/* XDP requires extra queues for XDP_TX */
>> +	if (curr_qp + xdp_qp > vi->max_queue_pairs) {
>> +		netdev_warn(dev, "request %i queues but max is %i\n",
>> +			    curr_qp + xdp_qp, vi->max_queue_pairs);
>> +		return -ENOMEM;
>> +	}
> 
> Can't we disable XDP_TX somehow? Many people might only want RX drop,
> and extra queues are not always there.
> 

Alexei, Daniel, any thoughts on this?

I know we were trying to claim some base level of feature support for
all XDP drivers. I am sympathetic to this argument though for DDOS we
do not need XDP_TX support. And virtio can become queue constrained
in some cases.

But, I do not want to silently degrade to RX mode and trying to check
this through the verifier appears challenging. And I'm not thrilled
about more knobs :/ Maybe an escape to force RX mode in sysfs or at
program load time would be OK?

I think this is an improvement that can go on my list along with LRO.

.John