On 11/10/17 19:57, Willem de Bruijn wrote: > On Wed, Oct 11, 2017 at 2:39 PM, Anton Ivanov > <anton.iva...@kot-begemot.co.uk> wrote: >> The check as now insists that the actual driver supports GSO_ROBUST, because >> we have marked the skb dodgy. >> >> The specific bit which does this check is in net_gso_ok() >> >> Now, lets's see how many Ethernet drivers set GSO_ROBUST. >> >> find drivers/net/ethernet -type f -name "*.[c,h]" -exec grep -H GSO_ROBUST >> {} \; >> >> That returns nothing in 4.x >> >> IMHO - af_packet allocates the skb, does all checks (and extra may be added) >> on the gso, why is this set dodgy in the first place? > It is set when the header has to be validated. > > The segmentation logic will validate and fixup gso_segs. See for > instance tcp_gso_segment: > > if (skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST)) { > /* Packet is from an untrusted source, reset gso_segs. */ > > skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len, mss); > > segs = NULL; > goto out; > } > > If the device would have the robust bit set and otherwise supports the > required features, fix up gso_segs and pass the large packet to the > device. > > Else it continues to the software gso path. > > Large packets generated with psock_txring_vnet.c pass this test. I
That test is indeed a different path - this goes via the tpacket_snd which allocs via sock_alloc_send_skb. That results in a non-fragged skb as it calls pskb after that with data_len = 0 asking for a contiguous one. My stuff is using sendmmsg which ends up via packet_snd which allocs via sock_alloc_send_pskb which is invoked in a way which always creates 2 segments - one for the linear section and one for the rest (and more if needed). It is faster than tpacket by the way (several times). As a comparison tap and other virtual drivers use sock_alloc_send_pskb with non-zero data length which results in multiple frags. The code in packet_snd is in fact identical with tap (+/- some cosmetic differences). That is the difference between the tests and that is why your test works and mine fails. Now, alloc-ing a 64k contiguous skb every time IMHO is wrong. So the logic in the xmit check at present works only because it is given only a very corner case for a GSO frame and tested versus it. It should work with the generic case which is what comes out of sock_alloc_send_pskb (same as in tap). A. > suspect that there is a subtle difference in the virtio_net_hdr fields > that that generates vs. your program. >