Subject: Re: [PATCH RFC net-next] virtio_net: Relax queue requirement for using XDP

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 26 Feb 2020 01:48:53 -0500

On Wed, Feb 26, 2020 at 11:00:40AM +0800, Jason Wang wrote:
> 
> On 2020/2/26 上午8:57, David Ahern wrote:
> > From: David Ahern <[email-masked]>
> > 
> > virtio_net currently requires extra queues to install an XDP program,
> > with the rule being twice as many queues as vcpus. From a host
> > perspective this means the VM needs to have 2*vcpus vhost threads
> > for each guest NIC for which XDP is to be allowed. For example, a
> > 16 vcpu VM with 2 tap devices needs 64 vhost threads.
> > 
> > The extra queues are only needed in case an XDP program wants to
> > return XDP_TX. XDP_PASS, XDP_DROP and XDP_REDIRECT do not need
> > additional queues. Relax the queue requirement and allow XDP
> > functionality based on resources. If an XDP program is loaded and
> > there are insufficient queues, then return a warning to the user
> > and if a program returns XDP_TX just drop the packet. This allows
> > the use of the rest of the XDP functionality to work without
> > putting an unreasonable burden on the host.
> > 
> > Cc: Jason Wang <[email-masked]>
> > Cc: Michael S. Tsirkin <[email-masked]>
> > Signed-off-by: David Ahern <[email-masked]>
> > ---
> >   drivers/net/virtio_net.c | 14 ++++++++++----
> >   1 file changed, 10 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 2fe7a3188282..2f4c5b2e674d 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -190,6 +190,8 @@ struct virtnet_info {
> >   	/* # of XDP queue pairs currently used by the driver */
> >   	u16 xdp_queue_pairs;
> > +	bool can_do_xdp_tx;
> > +
> >   	/* I like... big packets and I cannot lie! */
> >   	bool big_packets;
> > @@ -697,6 +699,8 @@ static struct sk_buff *receive_small(struct net_device *dev,
> >   			len = xdp.data_end - xdp.data;
> >   			break;
> >   		case XDP_TX:
> > +			if (!vi->can_do_xdp_tx)
> > +				goto err_xdp;
> 
> 
> I wonder if using spinlock to synchronize XDP_TX is better than dropping
> here?
> 
> Thanks

I think it's less a problem with locking, and more a problem
with queue being potentially full and XDP being unable to
transmit.

>From that POV just sharing the queue would already be better than just
an uncondiitonal drop, however I think this is not what XDP users came
to expect. So at this point, partitioning the queue might be reasonable.
When XDP attaches we could block until queue is mostly empty. However,
how exactly to partition the queue remains open.  Maybe it's reasonable
to limit number of RX buffers to achieve balance.

> 
> >   			stats->xdp_tx++;
> >   			xdpf = convert_to_xdp_frame(&xdp);
> >   			if (unlikely(!xdpf))
> > @@ -870,6 +874,8 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
> >   			}
> >   			break;
> >   		case XDP_TX:
> > +			if (!vi->can_do_xdp_tx)
> > +				goto err_xdp;
> >   			stats->xdp_tx++;
> >   			xdpf = convert_to_xdp_frame(&xdp);
> >   			if (unlikely(!xdpf))
> > @@ -2435,10 +2441,10 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
> >   	/* XDP requires extra queues for XDP_TX */
> >   	if (curr_qp + xdp_qp > vi->max_queue_pairs) {
> > -		NL_SET_ERR_MSG_MOD(extack, "Too few free TX rings available");
> > -		netdev_warn(dev, "request %i queues but max is %i\n",
> > -			    curr_qp + xdp_qp, vi->max_queue_pairs);
> > -		return -ENOMEM;
> > +		NL_SET_ERR_MSG_MOD(extack, "Too few free TX rings available; XDP_TX will not be allowed");
> > +		vi->can_do_xdp_tx = false;
> > +	} else {
> > +		vi->can_do_xdp_tx = true;
> >   	}
> >   	old_prog = rtnl_dereference(vi->rq[0].xdp_prog);