> -----Original Message----- > From: Dexuan Cui <decui@xxxxxxxxxxxxx> > Sent: Saturday, February 15, 2020 12:24 AM > To: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>; Stephen Hemminger > <sthemmin@xxxxxxxxxxxxx>; David S. Miller <davem@xxxxxxxxxxxxx>; > netdev@xxxxxxxxxxxxxxx; KY Srinivasan <kys@xxxxxxxxxxxxx>; linux- > kernel@xxxxxxxxxxxxxxx > Cc: linux-hyperv@xxxxxxxxxxxxxxx > Subject: Is it safe for a NIC driver to use all the 48 bytes of skb->cb? > > Hi, > It looks all the layers of drivers among the network stack can use the 48-byte > skb->cb array. Is there any rule how they should coordinate with each other? > > I noticed the last 16 bytes are used by struct skb_gso_cb: > > include/linux/skbuff.h: > struct skb_gso_cb { > union { > int mac_offset; > int data_offset; > }; > int encap_level; > __wsum csum; > __u16 csum_start; > }; > #define SKB_SGO_CB_OFFSET 32 > #define SKB_GSO_CB(skb) ((struct skb_gso_cb *)((skb)->cb + > SKB_SGO_CB_OFFSET)) > > Does this mean a low level NIC driver (e.g. hv_netvsc) should only use the first > 32 bytes? What if the upper layer network stack starts to take up more space in > the future? 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