net: skbuff: improve comment on checksumming (78ea85f1) · Commits · 戴 / test

include/linux/skbuff.h

+75 −56

Original line number	Diff line number	Diff line
		@@ -34,6 +34,77 @@
		#include <linux/netdev_features.h>
		#include <net/flow_keys.h>

		/* A. Checksumming of received packets by device.
		*
		* CHECKSUM_NONE:
		*
		* Device failed to checksum this packet e.g. due to lack of capabilities.
		* The packet contains full (though not verified) checksum in packet but
		* not in skb->csum. Thus, skb->csum is undefined in this case.
		*
		* CHECKSUM_UNNECESSARY:
		*
		* The hardware you're dealing with doesn't calculate the full checksum
		* (as in CHECKSUM_COMPLETE), but it does parse headers and verify checksums
		* for specific protocols e.g. TCP/UDP/SCTP, then, for such packets it will
		* set CHECKSUM_UNNECESSARY if their checksums are okay. skb->csum is still
		* undefined in this case though. It is a bad option, but, unfortunately,
		* nowadays most vendors do this. Apparently with the secret goal to sell
		* you new devices, when you will add new protocol to your host, f.e. IPv6 8)
		*
		* CHECKSUM_COMPLETE:
		*
		* This is the most generic way. The device supplied checksum of the _whole_
		* packet as seen by netif_rx() and fills out in skb->csum. Meaning, the
		* hardware doesn't need to parse L3/L4 headers to implement this.
		*
		* Note: Even if device supports only some protocols, but is able to produce
		* skb->csum, it MUST use CHECKSUM_COMPLETE, not CHECKSUM_UNNECESSARY.
		*
		* CHECKSUM_PARTIAL:
		*
		* This is identical to the case for output below. This may occur on a packet
		* received directly from another Linux OS, e.g., a virtualized Linux kernel
		* on the same host. The packet can be treated in the same way as
		* CHECKSUM_UNNECESSARY, except that on output (i.e., forwarding) the
		* checksum must be filled in by the OS or the hardware.
		*
		* B. Checksumming on output.
		*
		* CHECKSUM_NONE:
		*
		* The skb was already checksummed by the protocol, or a checksum is not
		* required.
		*
		* CHECKSUM_PARTIAL:
		*
		* The device is required to checksum the packet as seen by hard_start_xmit()
		* from skb->csum_start up to the end, and to record/write the checksum at
		* offset skb->csum_start + skb->csum_offset.
		*
		* The device must show its capabilities in dev->features, set up at device
		* setup time, e.g. netdev_features.h:
		*
		* NETIF_F_HW_CSUM - It's a clever device, it's able to checksum everything.
		* NETIF_F_IP_CSUM - Device is dumb, it's able to checksum only TCP/UDP over
		* IPv4. Sigh. Vendors like this way for an unknown reason.
		* Though, see comment above about CHECKSUM_UNNECESSARY. 8)
		* NETIF_F_IPV6_CSUM - About as dumb as the last one but does IPv6 instead.
		* NETIF_F_... - Well, you get the picture.
		*
		* CHECKSUM_UNNECESSARY:
		*
		* Normally, the device will do per protocol specific checksumming. Protocol
		* implementations that do not want the NIC to perform the checksum
		* calculation should use this flag in their outgoing skbs.
		*
		* NETIF_F_FCOE_CRC - This indicates that the device can do FCoE FC CRC
		* offload. Correspondingly, the FCoE protocol driver
		* stack should use CHECKSUM_UNNECESSARY.
		*
		* Any questions? No questions, good. --ANK
		*/

		/* Don't change this without changing skb_csum_unnecessary! */
		#define CHECKSUM_NONE 0
		#define CHECKSUM_UNNECESSARY 1
		@@ -54,58 +125,6 @@
		SKB_DATA_ALIGN(sizeof(struct sk_buff)) + \
		SKB_DATA_ALIGN(sizeof(struct skb_shared_info)))

		/* A. Checksumming of received packets by device.
		*
		* NONE: device failed to checksum this packet.
		* skb->csum is undefined.
		*
		* UNNECESSARY: device parsed packet and wouldbe verified checksum.
		* skb->csum is undefined.
		* It is bad option, but, unfortunately, many of vendors do this.
		* Apparently with secret goal to sell you new device, when you
		* will add new protocol to your host. F.e. IPv6. 8)
		*
		* COMPLETE: the most generic way. Device supplied checksum of _all_
		* the packet as seen by netif_rx in skb->csum.
		* NOTE: Even if device supports only some protocols, but
		* is able to produce some skb->csum, it MUST use COMPLETE,
		* not UNNECESSARY.
		*
		* PARTIAL: identical to the case for output below. This may occur
		* on a packet received directly from another Linux OS, e.g.,
		* a virtualised Linux kernel on the same host. The packet can
		* be treated in the same way as UNNECESSARY except that on
		* output (i.e., forwarding) the checksum must be filled in
		* by the OS or the hardware.
		*
		* B. Checksumming on output.
		*
		* NONE: skb is checksummed by protocol or csum is not required.
		*
		* PARTIAL: device is required to csum packet as seen by hard_start_xmit
		* from skb->csum_start to the end and to record the checksum
		* at skb->csum_start + skb->csum_offset.
		*
		* Device must show its capabilities in dev->features, set
		* at device setup time.
		* NETIF_F_HW_CSUM - it is clever device, it is able to checksum
		* everything.
		* NETIF_F_IP_CSUM - device is dumb. It is able to csum only
		* TCP/UDP over IPv4. Sigh. Vendors like this
		* way by an unknown reason. Though, see comment above
		* about CHECKSUM_UNNECESSARY. 8)
		* NETIF_F_IPV6_CSUM about as dumb as the last one but does IPv6 instead.
		*
		* UNNECESSARY: device will do per protocol specific csum. Protocol drivers
		* that do not want net to perform the checksum calculation should use
		* this flag in their outgoing skbs.
		* NETIF_F_FCOE_CRC this indicates the device can do FCoE FC CRC
		* offload. Correspondingly, the FCoE protocol driver
		* stack should use CHECKSUM_UNNECESSARY.
		*
		* Any questions? No questions, good. --ANK
		*/

		struct net_device;
		struct scatterlist;
		struct pipe_inode_info;

Admin message