Subject: Re: [RFC PATCH net-next] net/core: initial support for stacked dev feature toggles

From: Michal Kubecek <[email-masked]>
Date: Mon, 26 Oct 2015 10:42:13 +0100

On Fri, Oct 23, 2015 at 10:51:09PM -0700, Alexander Duyck wrote:
> On 10/23/2015 08:40 PM, Jarod Wilson wrote:
> >
> >+static netdev_features_t netdev_sync_upper_features(struct net_device *lower,
> >+	struct net_device *upper, netdev_features_t features)
> >+{
> >+	netdev_features_t want = upper->wanted_features & lower->hw_features;
> >+
> >+	if (!(upper->wanted_features & NETIF_F_LRO)
> >+	    && (features & NETIF_F_LRO)) {
> >+		netdev_info(lower, "Dropping LRO, upper dev %s has it off.\n",
> >+			   upper->name);
> >+		features &= ~NETIF_F_LRO;
> >+	} else if ((want & NETIF_F_LRO) && !(features & NETIF_F_LRO)) {
> >+		netdev_info(lower, "Keeping LRO, upper dev %s has it on.\n",
> >+			   upper->name);
> >+		features |= NETIF_F_LRO;
> >+	}
> >+
> >+	return features;
> >+}
> >+
> 
> I'd say to drop the second half of this statement.  LRO is a feature
> that should be enabled explicitly per interface.  If someone enables
> LRO on the master they may only want it on one interface.  The fact
> is there are some implementations of LRO that work better than
> others so you want to give the end user the option to mix and match.

Agreed. IMHO it makes sense to allow setups with LRO disabled on some
slaves and enabled on other.

Also, the logic seems to only consider the 1 upper : N lower scheme
(bond, team) but we also have N upper : 1 lower setups (vlan, macvlan).
For these, there is no way to propagate both 0 and 1 down as this would
result in a conflict.

> >+static void netdev_sync_lower_features(struct net_device *upper,
> >+	struct net_device *lower, netdev_features_t features)
> >+{
> >+	netdev_features_t want = features & lower->hw_features;
> >+
> >+	if (!(features & NETIF_F_LRO) && (lower->features & NETIF_F_LRO)) {
> >+		netdev_info(upper, "Disabling LRO on lower dev %s.\n",
> >+			   lower->name);
> >+		upper->wanted_features &= ~NETIF_F_LRO;
> >+		lower->wanted_features &= ~NETIF_F_LRO;
> >+		netdev_update_features(lower);
> >+		if (unlikely(lower->features & NETIF_F_LRO))
> >+			netdev_WARN(upper, "failed to disable LRO on %s!\n",
> >+				    lower->name);
> >+	} else if ((want & NETIF_F_LRO) && !(lower->features & NETIF_F_LRO)) {
> >+		netdev_info(upper, "Enabling LRO on lower dev %s.\n",
> >+			   lower->name);
> >+		upper->wanted_features |= NETIF_F_LRO;
> >+		lower->wanted_features |= NETIF_F_LRO;
> >+		netdev_update_features(lower);
> >+		if (unlikely(!(lower->features & NETIF_F_LRO)))
> >+			netdev_WARN(upper, "failed to enable LRO on %s!\n",
> >+				    lower->name);
> >+	}
> >+}
> >+
> 
> Same thing here.  If a lower dev has it disabled then leave it
> disabled.  I believe your goal is to make it so that
> dev_disable_lro() can shut down LRO when it is making packets in the
> data-path unusable.

This is already the case since commit fbe168ba91f7 ("net: generic
dev_disable_lro() stacked device handling"). That commit makes sure
dev_disable_lro() is propagated down the stack and also makes sure new
slaves added to a bond/team with LRO disabled have it disabled too.

What it does not do is propagating LRO disabling down if it is disabled
in ways that do not call dev_disable_lro() (e.g. via ethtool). I'm not
sure if this should be done or not, both options have their pros and
cons. However, I believe enabling LRO shouldn't be propagated down.

                                                         Michal Kubecek