tpacket_rcv() does
if (sk->sk_type == SOCK_DGRAM) {
macoff = netoff = TPACKET_ALIGN(po->tp_hdrlen) + 16 +
po->tp_reserve;
} else {
unsigned int maclen = skb_network_offset(skb);
netoff = TPACKET_ALIGN(po->tp_hdrlen +
(maclen < 16 ? 16 : maclen)) +
po->tp_reserve;
if (po->has_vnet_hdr) {
netoff += sizeof(struct virtio_net_hdr);
do_vnet = true;
}
macoff = netoff - maclen;
}
1) What is the "+ 16" in the SOCK_DGRAM case? Is it reserving space for
libpcap to put a "Linux cooked mode" header in before the packet data? If so,
that should probably be mentioned in a comment, or a #define should be used.
2) The documentation speaks of putting the raw packet data on a 16-byte
boundary.
2a) Is this to avoid the "If the data is not aligned on a 16-Byte boundary,
then whenever a store crosses a cache-line boundary there is typically a
stall." issue mentioned on
https://software.intel.com/en-us/forums/intel-isa-extensions/topic/709279 ?
2b) Does this mean that the argument to the PACKET_RESERVE socket option should
be a multiple of 16? If so, that should probably be mentioned in the packet(7)
man page.
3) In the non-SOCK_DGRAM case, why does it use 16 instead of a
less-than-16-bytes maclen?
