Subject: Re: [V6 PATCH] virtio-net: send gratuitous packets when needed

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 4 Apr 2012 10:49:37 +0300

On Wed, Mar 28, 2012 at 01:44:28PM +0800, Jason Wang wrote:
> As hypervior does not have the knowledge of guest network configuration, it's
> better to ask guest to send gratuitous packets when needed.
> 
> Guest tests VIRTIO_NET_S_ANNOUNCE bit during config change interrupt and when it
> is set, a workqueue is scheduled to send gratuitous packet through
> NETDEV_NOTIFY_PEERS. This feature is negotiated through bit
> VIRTIO_NET_F_GUEST_ANNOUNCE.
> 
> Changes from v5:
> - notify the chain before acking the link annoucement
> - ack the link announcement notification through control vq
> 
> Changes from v4:
> - typos
> - handle workqueue unconditionally
> - move VIRTIO_NET_S_ANNOUNCE to bit 8 to separate rw bits from ro bits
> 
> Changes from v3:
> - cancel the workqueue during freeze
> 
> Changes from v2:
> - fix the race between unregister_dev() and workqueue
> 
> Signed-off-by: Jason Wang <[email-masked]>

I think this needs some fixes. See below.
Thanks.

> ---
>  drivers/net/virtio_net.c   |   32 +++++++++++++++++++++++++++++++-
>  include/linux/virtio_net.h |   13 +++++++++++++
>  2 files changed, 44 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 4880aa8..0f60da7 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -72,6 +72,9 @@ struct virtnet_info {
>  	/* Work struct for refilling if we run low on memory. */
>  	struct delayed_work refill;
>  
> +	/* Work struct for sending gratuitous packets. */

A bit confusing: it does not send anything itself.
A better comment 'for announcing the existence of device
on the network'.

> +	struct work_struct announce;
> +
>  	/* Chain pages by the private ptr. */
>  	struct page *pages;
>  
> @@ -781,12 +784,30 @@ static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd,
>  	return status == VIRTIO_NET_OK;
>  }
>  
> +static void virtnet_ack_link_announce(struct virtnet_info *vi)
> +{
> +	if (!virtnet_send_command(vi, VIRTIO_NET_CTRL_ANNOUNCE,
> +				  VIRTIO_NET_CTRL_ANNOUNCE_ACK, NULL,
> +				  0, 0)) {

This can run in parallel with other commands. That's pretty bad -
will corrupt the cvq.
Take rtnl lock around calls to this function?

> +		dev_warn(&vi->dev->dev, "Failed to ack link nnounce.\n");

announce

> +	}

Better to drop {} around a single statement.

> +}
> +
> +static void announce_work(struct work_struct *work)
> +{
> +	struct virtnet_info *vi = container_of(work, struct virtnet_info,
> +					       announce);
> +	netif_notify_peers(vi->dev);
> +	virtnet_ack_link_announce(vi);
> +}
> +
>  static int virtnet_close(struct net_device *dev)
>  {
>  	struct virtnet_info *vi = netdev_priv(dev);
>  
>  	/* Make sure refill_work doesn't re-enable napi! */
>  	cancel_delayed_work_sync(&vi->refill);
> +	cancel_work_sync(&vi->announce);

I think that a config change event can trigger after this point,
so cancel here won't be effective. But why do it here?
virtnet_remove not a better place? We can do it
after remove_vq_common.

>  	napi_disable(&vi->napi);
>  
>  	return 0;
> @@ -962,11 +983,17 @@ static void virtnet_update_status(struct virtnet_info *vi)
>  		return;
>  
>  	/* Ignore unknown (future) status bits */
> -	v &= VIRTIO_NET_S_LINK_UP;
> +	v &= VIRTIO_NET_S_LINK_UP | VIRTIO_NET_S_ANNOUNCE;

The announce bit in vi->status is always clear,
so this is IMO confusing. I would do:

	if (v & VIRTIO_NET_S_ANNOUNCE) {
		schedule
	}

	v &= VIRTIO_NET_S_LINK_UP;

and the rest of the logic is not necessary then.

Also, this might run extra ack announce commands after
VIRTIO_NET_S_ANNOUNCE bit is clear. The spec
isn't clear on whether this is legal.

It would be very hard to fix this, so let's add a comment
stating that it's legal, and clarify the spec
in any case.

>  
>  	if (vi->status == v)
>  		return;
>  
> +	if (v & VIRTIO_NET_S_ANNOUNCE) {
> +		v &= ~VIRTIO_NET_S_ANNOUNCE;
> +		if (v & VIRTIO_NET_S_LINK_UP)
> +			schedule_work(&vi->announce);

I think we really want an nrt wq here - if this triggers
multiple times there's no good reason to try and run
the ack command many times in parallel.

> +	}
> +
>  	vi->status = v;
>  
>  	if (vi->status & VIRTIO_NET_S_LINK_UP) {

> @@ -1076,6 +1103,7 @@ static int virtnet_probe(struct virtio_device *vdev)
>  		goto free;
>  
>  	INIT_DELAYED_WORK(&vi->refill, refill_work);
> +	INIT_WORK(&vi->announce, announce_work);
>  	sg_init_table(vi->rx_sg, ARRAY_SIZE(vi->rx_sg));
>  	sg_init_table(vi->tx_sg, ARRAY_SIZE(vi->tx_sg));
>  
> @@ -1187,6 +1215,7 @@ static int virtnet_freeze(struct virtio_device *vdev)
>  	virtqueue_disable_cb(vi->svq);
>  	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_CTRL_VQ))
>  		virtqueue_disable_cb(vi->cvq);
> +	cancel_work_sync(&vi->announce);

A config change event can trigger after this point,
so cancel here won't be effective.
Possibly, we need state like config_enable in the block
device.

Also, what exactly will happen on suspend?
As we reset, ANNOUCE bit will be clear so -
do we forget to announce? Probably not good ...

>  
>  	netif_device_detach(vi->dev);
>  	cancel_delayed_work_sync(&vi->refill);
> @@ -1233,6 +1262,7 @@ static unsigned int features[] = {
>  	VIRTIO_NET_F_GUEST_ECN, VIRTIO_NET_F_GUEST_UFO,
>  	VIRTIO_NET_F_MRG_RXBUF, VIRTIO_NET_F_STATUS, VIRTIO_NET_F_CTRL_VQ,
>  	VIRTIO_NET_F_CTRL_RX, VIRTIO_NET_F_CTRL_VLAN,
> +	VIRTIO_NET_F_GUEST_ANNOUNCE,
>  };
>  
>  static struct virtio_driver virtio_net_driver = {
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index 970d5a2..383e8a0 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -49,8 +49,10 @@
>  #define VIRTIO_NET_F_CTRL_RX	18	/* Control channel RX mode support */
>  #define VIRTIO_NET_F_CTRL_VLAN	19	/* Control channel VLAN filtering */
>  #define VIRTIO_NET_F_CTRL_RX_EXTRA 20	/* Extra RX mode control support */
> +#define VIRTIO_NET_F_GUEST_ANNOUNCE 21  /* Guest can send gratituous packet */

Rusty aligned the comments using tabs so let's do it here too?
A better comment 'can announce device on the network'.

>  
>  #define VIRTIO_NET_S_LINK_UP	1	/* Link is up */
> +#define VIRTIO_NET_S_ANNOUNCE   2	/* Announcement is needed */

why 3 spaces before the value here?

>  
>  struct virtio_net_config {
>  	/* The config defining mac address (if VIRTIO_NET_F_MAC) */
> @@ -152,4 +154,15 @@ struct virtio_net_ctrl_mac {
>   #define VIRTIO_NET_CTRL_VLAN_ADD             0
>   #define VIRTIO_NET_CTRL_VLAN_DEL             1
>  
> +/*
> + * Control link announce acknowledgement
> + *
> + * The command VIRTIO_NET_CTRL_ANNOUNCE_ACK is used to indicate that
> + * driver has recevied the notification and device would clear the

s/recevied/received/

A bit clearer to replace 'and' with ; here.

> + * VIRTIO_NET_S_ANNOUNCE bit in the status filed after it received

s/filed/field/
s/received/receives/

> + * this command.
> + */
> +#define VIRTIO_NET_CTRL_ANNOUNCE       3
> + #define VIRTIO_NET_CTRL_ANNOUNCE_ACK         0
> +
>  #endif /* _LINUX_VIRTIO_NET_H */
> 
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