Subject: Re: [PATCH net-next 1/2] macvtap: Let TUNSETOFFLOAD actually controll offload features.

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 19 Jun 2013 18:55:49 +0300

On Wed, Jun 19, 2013 at 11:47:42AM -0400, Vlad Yasevich wrote:
> On 06/19/2013 11:16 AM, Michael S. Tsirkin wrote:
> >On Wed, Jun 19, 2013 at 10:47:51AM -0400, Vlad Yasevich wrote:
> >>When the user issues TUNSETOFFLOAD ioctl, macvtap does not do
> >>anything other then to verify arguments.  This patch adds
> >>functionality to allow users to actually control offload features.
> >>NETIF_F_GSO and NETIF_F_GRO are always on, but the rest of the
> >>features can be controlled.
> >>
> >>Signed-off-by: Vlad Yasevich <[email-masked]>
> >>---
> >>  drivers/net/macvlan.c      |  9 +++++++++
> >>  drivers/net/macvtap.c      | 41 ++++++++++++++++++++++++++++++++++++++++-
> >>  include/linux/if_macvlan.h |  1 +
> >>  3 files changed, 50 insertions(+), 1 deletion(-)
> >>
> >>diff --git a/drivers/net/macvlan.c b/drivers/net/macvlan.c
> >>index edfddc5..fa47415 100644
> >>--- a/drivers/net/macvlan.c
> >>+++ b/drivers/net/macvlan.c
> >>@@ -638,6 +638,14 @@ static int macvlan_ethtool_get_settings(struct net_device *dev,
> >>  	return __ethtool_get_settings(vlan->lowerdev, cmd);
> >>  }
> >>
> >>+static netdev_features_t macvlan_fix_features(struct net_device *dev,
> >>+					      netdev_features_t features)
> >>+{
> >>+	struct macvlan_dev *vlan = netdev_priv(dev);
> >>+
> >>+	return (features & vlan->set_features) | (features & ~MACVLAN_FEATURES);
> >
> >A bit clearer as
> >
> >>+	return features & (vlan->set_features | ~MACVLAN_FEATURES);
> 
> OK
> 
> >
> >>+}
> >>+
> >>  static const struct ethtool_ops macvlan_ethtool_ops = {
> >>  	.get_link		= ethtool_op_get_link,
> >>  	.get_settings		= macvlan_ethtool_get_settings,
> >>@@ -651,6 +659,7 @@ static const struct net_device_ops macvlan_netdev_ops = {
> >>  	.ndo_stop		= macvlan_stop,
> >>  	.ndo_start_xmit		= macvlan_start_xmit,
> >>  	.ndo_change_mtu		= macvlan_change_mtu,
> >>+	.ndo_fix_features	= macvlan_fix_features,
> >>  	.ndo_change_rx_flags	= macvlan_change_rx_flags,
> >>  	.ndo_set_mac_address	= macvlan_set_mac_address,
> >>  	.ndo_set_rx_mode	= macvlan_set_mac_lists,
> >>diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> >>index 5a76f20..09f0b1f 100644
> >>--- a/drivers/net/macvtap.c
> >>+++ b/drivers/net/macvtap.c
> >>@@ -976,6 +976,45 @@ static int macvtap_ioctl_set_queue(struct file *file, unsigned int flags)
> >>  	return ret;
> >>  }
> >>
> >>+static int set_offload(struct macvtap_queue *q, unsigned long arg)
> >>+{
> >>+	struct macvlan_dev *vlan;
> >>+	netdev_features_t features = NETIF_F_GSO|NETIF_F_GRO;
> >>+	int err = 0;
> >>+
> >>+	if (arg & TUN_F_CSUM) {
> >>+		features = NETIF_F_HW_CSUM;
> >>+
> >>+		if (arg & (TUN_F_TSO4 | TUN_F_TSO6)) {
> >>+			if (arg & TUN_F_TSO_ECN)
> >>+				features |= NETIF_F_TSO_ECN;
> >>+			if (arg & TUN_F_TSO4)
> >>+				features |= NETIF_F_TSO;
> >>+			if (arg & TUN_F_TSO6)
> >>+				features |= NETIF_F_TSO6;
> >>+		}
> >>+
> >>+		if (arg & TUN_F_UFO)
> >>+			features |= NETIF_F_UFO;
> >
> >Hmm this looks strange. The meaning of offloads
> >with tun is exactly the reverse from vlan/macvtap.
> >
> >For example, assume that you disable TSO.
> >For tun this means: "don't send TSO packets to userspace".
> >What this patch makes it mean for macvtap is
> >"don't send TSO packets from userspace on the network".
> >
> 
> Isn't a user space write to TUN exactly the same as
> a user space write to macvtap?  It looks to me like the
> are and so features for them would work the same way.
> 
> macvlan and macvtap would be different, but I think that's
> to be expected.

They aren't the same.

Userspace write on tun causes a packet to be *received* from tun.
Userspace write on macvtap causes a packet to be *transmitted*
on macvlan.

> >So, userspace using this ioctl
> >to control tun would get a surprising result.
> 
> By surprising do you mean that if user space writes
> a TSO packet to a macvtap where TSO is disabled, the TSO
> packet is still sent to the network?

No.
tun offloads only control packets send to userspace.
When I disable TSO on tun this means
don't send *me* TSO packets. Instead, you try to
mess with packets *received* from me and being
sent outside.

> 
> >
> >>+	}
> >>+
> >>+	rtnl_lock();
> >>+	rcu_read_lock_bh();
> >>+	vlan = rcu_dereference_bh(q->vlan);
> >>+	if (!vlan) {
> >>+		err = -ENOLINK;
> >>+		goto unlock;
> >>+	}
> >>+
> >>+	vlan->set_features = features;
> >>+	netdev_update_features(vlan->dev);
> >>+
> >>+unlock:
> >>+	rcu_read_unlock_bh();
> >>+	rtnl_unlock();
> >>+	return err;
> >>+}
> >>+
> >>  /*
> >>   * provide compatibility with generic tun/tap interface
> >>   */
> >>@@ -1062,7 +1101,7 @@ static long macvtap_ioctl(struct file *file, unsigned int cmd,
> >>  			 got enabled for forwarded frames */
> >>  		if (!(q->flags & IFF_VNET_HDR))
> >>  			return  -EINVAL;
> >>-		return 0;
> >>+		return set_offload(q, arg);
> >>
> >>  	default:
> >>  		return -EINVAL;
> >>diff --git a/include/linux/if_macvlan.h b/include/linux/if_macvlan.h
> >>index f49a9f6..e446e82 100644
> >>--- a/include/linux/if_macvlan.h
> >>+++ b/include/linux/if_macvlan.h
> >>@@ -65,6 +65,7 @@ struct macvlan_dev {
> >>
> >>  	DECLARE_BITMAP(mc_filter, MACVLAN_MC_FILTER_SZ);
> >>
> >>+	netdev_features_t	set_features;
> >>  	enum macvlan_mode	mode;
> >>  	u16			flags;
> >>  	int (*receive)(struct sk_buff *skb);
> >>--
> >>1.8.1.4