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

From: Jason Wang <[email-masked]>
Date: Thu, 05 Apr 2012 13:56:15 +0800

On 04/04/2012 03:49 PM, Michael S. Tsirkin wrote:
> 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?

It's only used by callbacks of netif_notify_peers(), so rtnl lock have 
already been hold. Add a comment to clarify this?
>> +		dev_warn(&vi->dev->dev, "Failed to ack link nnounce.\n");
> announce

Sorry for the typos, would double check in the future.
>> +	}
> 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.

Sure.
>>   	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.

How about delay the clearing of VIRTIO_NET_S_ANNOUNCE bit in the 
workqueue and protect the updating with 
spinlock_irqsave()/spinloc_irqrestore()? Then later notification would 
be suppressed if the sending is not completed.
>>
>>   	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.

Yes.
>> +	}
>> +
>>   	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.

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

This problem should be the same as normal suspending/resuming, if user 
want to announce the link they should use arp_notify instead.
>
>>
>>   	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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