> -----Original Message----- > From: Dexuan Cui <decui@xxxxxxxxxxxxx> > Sent: Saturday, February 15, 2020 1:04 PM > 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: RE: Is it safe for a NIC driver to use all the 48 bytes of skb->cb? > > > 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? skb_cow_head() only expands the headroom if it is not enough, in case some upper layer path didn't reserve enough. > 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. Unrelated to MMU. It just copies some data to make room for writing. Thanks, - Haiyang
