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diff --git a/Documentation/networking/netdev-features.rst b/Documentation/networking/netdev-features.rst new file mode 100644 index 000000000..a2d7d7160 --- /dev/null +++ b/Documentation/networking/netdev-features.rst @@ -0,0 +1,184 @@ +.. SPDX-License-Identifier: GPL-2.0 + +===================================================== +Netdev features mess and how to get out from it alive +===================================================== + +Author: + Michał Mirosław <mirq-linux@rere.qmqm.pl> + + + +Part I: Feature sets +==================== + +Long gone are the days when a network card would just take and give packets +verbatim. Today's devices add multiple features and bugs (read: offloads) +that relieve an OS of various tasks like generating and checking checksums, +splitting packets, classifying them. Those capabilities and their state +are commonly referred to as netdev features in Linux kernel world. + +There are currently three sets of features relevant to the driver, and +one used internally by network core: + + 1. netdev->hw_features set contains features whose state may possibly + be changed (enabled or disabled) for a particular device by user's + request. This set should be initialized in ndo_init callback and not + changed later. + + 2. netdev->features set contains features which are currently enabled + for a device. This should be changed only by network core or in + error paths of ndo_set_features callback. + + 3. netdev->vlan_features set contains features whose state is inherited + by child VLAN devices (limits netdev->features set). This is currently + used for all VLAN devices whether tags are stripped or inserted in + hardware or software. + + 4. netdev->wanted_features set contains feature set requested by user. + This set is filtered by ndo_fix_features callback whenever it or + some device-specific conditions change. This set is internal to + networking core and should not be referenced in drivers. + + + +Part II: Controlling enabled features +===================================== + +When current feature set (netdev->features) is to be changed, new set +is calculated and filtered by calling ndo_fix_features callback +and netdev_fix_features(). If the resulting set differs from current +set, it is passed to ndo_set_features callback and (if the callback +returns success) replaces value stored in netdev->features. +NETDEV_FEAT_CHANGE notification is issued after that whenever current +set might have changed. + +The following events trigger recalculation: + 1. device's registration, after ndo_init returned success + 2. user requested changes in features state + 3. netdev_update_features() is called + +ndo_*_features callbacks are called with rtnl_lock held. Missing callbacks +are treated as always returning success. + +A driver that wants to trigger recalculation must do so by calling +netdev_update_features() while holding rtnl_lock. This should not be done +from ndo_*_features callbacks. netdev->features should not be modified by +driver except by means of ndo_fix_features callback. + + + +Part III: Implementation hints +============================== + + * ndo_fix_features: + +All dependencies between features should be resolved here. The resulting +set can be reduced further by networking core imposed limitations (as coded +in netdev_fix_features()). For this reason it is safer to disable a feature +when its dependencies are not met instead of forcing the dependency on. + +This callback should not modify hardware nor driver state (should be +stateless). It can be called multiple times between successive +ndo_set_features calls. + +Callback must not alter features contained in NETIF_F_SOFT_FEATURES or +NETIF_F_NEVER_CHANGE sets. The exception is NETIF_F_VLAN_CHALLENGED but +care must be taken as the change won't affect already configured VLANs. + + * ndo_set_features: + +Hardware should be reconfigured to match passed feature set. The set +should not be altered unless some error condition happens that can't +be reliably detected in ndo_fix_features. In this case, the callback +should update netdev->features to match resulting hardware state. +Errors returned are not (and cannot be) propagated anywhere except dmesg. +(Note: successful return is zero, >0 means silent error.) + + + +Part IV: Features +================= + +For current list of features, see include/linux/netdev_features.h. +This section describes semantics of some of them. + + * Transmit checksumming + +For complete description, see comments near the top of include/linux/skbuff.h. + +Note: NETIF_F_HW_CSUM is a superset of NETIF_F_IP_CSUM + NETIF_F_IPV6_CSUM. +It means that device can fill TCP/UDP-like checksum anywhere in the packets +whatever headers there might be. + + * Transmit TCP segmentation offload + +NETIF_F_TSO_ECN means that hardware can properly split packets with CWR bit +set, be it TCPv4 (when NETIF_F_TSO is enabled) or TCPv6 (NETIF_F_TSO6). + + * Transmit UDP segmentation offload + +NETIF_F_GSO_UDP_L4 accepts a single UDP header with a payload that exceeds +gso_size. On segmentation, it segments the payload on gso_size boundaries and +replicates the network and UDP headers (fixing up the last one if less than +gso_size). + + * Transmit DMA from high memory + +On platforms where this is relevant, NETIF_F_HIGHDMA signals that +ndo_start_xmit can handle skbs with frags in high memory. + + * Transmit scatter-gather + +Those features say that ndo_start_xmit can handle fragmented skbs: +NETIF_F_SG --- paged skbs (skb_shinfo()->frags), NETIF_F_FRAGLIST --- +chained skbs (skb->next/prev list). + + * Software features + +Features contained in NETIF_F_SOFT_FEATURES are features of networking +stack. Driver should not change behaviour based on them. + + * LLTX driver (deprecated for hardware drivers) + +NETIF_F_LLTX is meant to be used by drivers that don't need locking at all, +e.g. software tunnels. + +This is also used in a few legacy drivers that implement their +own locking, don't use it for new (hardware) drivers. + + * netns-local device + +NETIF_F_NETNS_LOCAL is set for devices that are not allowed to move between +network namespaces (e.g. loopback). + +Don't use it in drivers. + + * VLAN challenged + +NETIF_F_VLAN_CHALLENGED should be set for devices which can't cope with VLAN +headers. Some drivers set this because the cards can't handle the bigger MTU. +[FIXME: Those cases could be fixed in VLAN code by allowing only reduced-MTU +VLANs. This may be not useful, though.] + +* rx-fcs + +This requests that the NIC append the Ethernet Frame Checksum (FCS) +to the end of the skb data. This allows sniffers and other tools to +read the CRC recorded by the NIC on receipt of the packet. + +* rx-all + +This requests that the NIC receive all possible frames, including errored +frames (such as bad FCS, etc). This can be helpful when sniffing a link with +bad packets on it. Some NICs may receive more packets if also put into normal +PROMISC mode. + +* rx-gro-hw + +This requests that the NIC enables Hardware GRO (generic receive offload). +Hardware GRO is basically the exact reverse of TSO, and is generally +stricter than Hardware LRO. A packet stream merged by Hardware GRO must +be re-segmentable by GSO or TSO back to the exact original packet stream. +Hardware GRO is dependent on RXCSUM since every packet successfully merged +by hardware must also have the checksum verified by hardware. |