On Thu, 22 Oct 2020 12:59:45 -0700 Xie He wrote: > On Thu, Oct 22, 2020 at 8:22 AM Jakub Kicinski <k...@kernel.org> wrote: > > > > Are most of these drivers using skb_padto()? Is that the reason they > > can't be sharing the SKB? > > Yes, I think if a driver calls skb_pad / skb_padto / skb_put_padto / > eth_skb_pad, the driver can't accept shared skbs because it may modify > the skbs. > > > I think the IFF_TX_SKB_SHARING flag is only used by pktgen, so perhaps > > we can make sure pktgen doesn't generate skbs < dev->min_mtu, and then > > the drivers won't pad? > > Yes, I see a lot of drivers just want to pad the skb to ETH_ZLEN, or > just call eth_skb_pad. In this case, requiring the shared skb to be at > least dev->min_mtu long can solve the problem for these drivers. > > But I also see some drivers that want to pad the skb to a strange > length, and don't set their special min_mtu to match this length. For > example: > > drivers/net/ethernet/packetengines/yellowfin.c wants to pad the skb to > a dynamically calculated value. > > drivers/net/ethernet/ti/cpsw.c, cpsw_new.c and tlan.c want to pad the > skb to macro defined values. > > drivers/net/ethernet/intel/iavf/iavf_txrx.c wants to pad the skb to > IAVF_MIN_TX_LEN (17). > > drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c wants to pad the skb to 17.
Hm, I see, that would be a slight loss of functionality if we started requiring 64B, for example, while the driver could in practice xmit 17B frames (would matter only to VFs, but nonetheless). > Another solution I can think of is to add a "skb_shared" check to > "__skb_pad", so that if __skb_pad encounters a shared skb, it just > returns an error. The driver would think this is a memory allocation > failure. This way we can ensure shared skbs are not modified. I'm not sure if we want to be adding checks to __skb_pad() to handle what's effectively a pktgen specific condition. We could create a new field in struct netdevice for min_frame_len, but I think your patch is the simplest solution. Let's see if anyone objects. BTW it seems like there is more drivers which will need the flag cleared, e.g. drivers/net/ethernet/broadcom/bnxt/bnxt.c?
