> From: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx> > Sent: Saturday, February 15, 2020 7:20 AM > To: Dexuan Cui <decui@xxxxxxxxxxxxx>; Stephen Hemminger > > According to the comments in skbuff.h below, it is the responsibility of the > owning layer to make a SKB clone, if it wants to keep the data across layers. > So, every layer can still use all of the 48 bytes. > > /* > * This is the control buffer. It is free to use for every > * layer. Please put your private variables there. If you > * want to keep them across layers you have to do a skb_clone() > * first. This is owned by whoever has the skb queued ATM. > */ > char cb[48] __aligned(8); > > > Now hv_netvsc assumes it can use all of the 48-bytes, though it uses only > > 20 bytes, but just in case the struct hv_netvsc_packet grows to >32 bytes in > the > > future, should we change the BUILD_BUG_ON() in netvsc_start_xmit() to > > BUILD_BUG_ON(sizeof(struct hv_netvsc_packet) > SKB_SGO_CB_OFFSET); ? > > Based on the explanation above, the existing hv_netvsc code is correct. > > Thanks, > - Haiyang Got it. So if the upper layer saves something in the cb, it must do a skb_clone() and pass the new skb to hv_netvsc. hv_netvsc is the lowest layer in the network stack, so it can use all the 48 bytes without calling skb_clone(). BTW, now I happen to have a different question: in netvsc_probe() we have net->needed_headroom = RNDIS_AND_PPI_SIZE; I think this means when the network stack (ARP, IP, ICMP, TCP, UDP,etc) passes a skb to hv_netvsc, the skb's headroom is increased by an extra size of net->needed_headroom, right? Then in netvsc_xmit(), why do we still need to call skb_cow_head(skb, RNDIS_AND_PPI_SIZE)? -- this looks unnecessary to me? PS, what does the "cow" here mean? Copy On Write? It looks skb_cow_head() just copies the data (if necessary) and it has nothing to do with the write-protection in the MMU code. Thanks, Dexuan
