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

From: Jarod Wilson
Date: Fri Oct 30 2015 - 12:36:04 EST

Alexander Duyck wrote:
On 10/23/2015 08:40 PM, Jarod Wilson wrote:
There are some netdev features that make little sense to toggle on and
off in a stacked device setup on only one device in the stack. The prime
example is a bonded connection, where it really doesn't make sense to
disable LRO on the master, but not on any of the slaves, nor does it
really make sense to be able to shut LRO off on a slave when its still
enabled on the master.

The strategy here is to add a section near the end of
netdev_fix_features() that looks for upper and lower netdevs, then make
sure certain feature flags match both up and down the stack. At present,
only the LRO flag is included.
+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. There is
no need to make this an all or nothing scenario. We can let the stack
slam things down with dev_disable_lro() and then if a user so desires
they can come back through and enable LRO more selectively if they for
instance have an interface that can do a smarter job of putting together
frames that could be routed.

You could probably look at doing something like this for RXCSUM as well.
The general idea is that if an upper device has it off then the value
has to be off. For example if RXCSUM is off in a upper device and LRO is
enabled on the lower device there is a good chance that the upper device
will report checksum errors since most LRO implementations don't
recalculate the checksum. If RXCSUM is forced down to the lower device
hopefully its fix_features will know this and disable LRO on that device
when the RXCSUM is disabled on it.

Yeah, I was thinking there might be more flags to treat the same way, just wanted to hammer out the plausibility of doing it at all first. I can add RXCSUM to v2, or just wait until there's something that people might consider merge-worthy before worrying about additional flags. From what I've seen, most device's fix_features are reasonably intelligent about allowing/disallowing certain flag combos, so this does look pretty safe at a glance, and if a specific device tips over, it probably needs to be fixed in the device's driver anyway.

Jarod Wilson

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