> On Wed, 12 Aug 2026 10:33:19 +0000 Chia-Yu Chang (Nokia) wrote: > > > > - /* This indicates the tcp segment has CWR set. */ > > > > + /* For TX, this indicates that the first TCP segment has CWR set, > > > > and > > > > + * any subsequent segment in the same skb has CWR cleared. This > > > > flag > > > > + * must not be used in RX, because the connection to which the > > > > segment > > > > + * belongs is not tracked to use RFC3168 or AccECN. Using RFC3168 > > > > ECN > > > > + * offload may clear CWR and corrupt ACE signal (CWR is part of > > > > it). > > > > + * Instead, SKB_GSO_TCP_ACCECN shall be used to avoid CWR > > > > corruption. > > > > + */ > > > > > > I still can't wrap my head around this TBH. > > > > > > SKB_GSO_TCP_ECN means RFC3168 > > > SKB_GSO_TCP_ACCECN means AccECN > > > > > > If the HW can correctly detect cwr on first frame and then no cwr and > > > report that as ECN/RFC3168 - what's the problem? TSO will produce the > > > exact expected segment sequence. > > > > > > Is the program that if we re-GRO that frame in SW we end up with > > > ECN+ACCECN on the same skb? > > > > Yes, this is the problem. > > The HW does not know whether the received packets belong to an RFC3168 ECN > > flow or an AccECN flow on the RX path. > > For example, HW GRO may set SKB_GSO_TCP_ECN after observing that the first > > packet has CWR=1: > > > > +===================+==========+=================+================+ > > | Packet id | CWR flag | Flag | Flushed as SKB | > > +===================+==========+=================+================+ > > | 0 | 1 | SKB_GSO_TCP_ECN | 0 | > > | 1 | 0 | - | 0 | > > | 2 | 1 | - | 0 | > > | 3 | 1 | - | 1 | > > +===================+==========+=================+================+ > > > > If the aggregated skb is forwarded through a device using GSO, e.g., > > HW RX (GRO) -> veth TX (GSO), the SKB_GSO_TCP_ECN applies RFC3168 > > semantics. This means that only the 1st segment keeps the CWR flag > > while all subsequent segments have CWR cleared: > > > > +===================+==========+ > > | Packet id | CWR flag | > > +===================+==========+ > > | 0 | 1 | > > | 1 | 0 | > > | 2 | 0 | > > | 3 | 0 | > > +===================+==========+ > > > > This behavior is ok for RFC3168, since CWR is expected to appear only > > once. However, for AccECN, CWR is part of the ACE signal and must be > > preserved across all segments. > > But this would be obviously a buggy HW-GRO implementation. > The rules for HW-GRO RFC3168 are -- ignore CWR on first segment (host > responsible for populating SKB_GSO_TCP_ECN), and CWR _must be 0_ for all > subsequent segments. > > > In the example above, the original CWR sequence was 1,0,1,1. > > But after re-segmentation it becomes: 1,0,0,0. > > This is why SKB_GSO_TCP_ECN should not be used in RX/GRO paths. > > We have extensive gro tests under > tools/testing/selftests/drivers/net/gro.py > > If you want to catch bad devices - add appropriate test cases there. >
I added a test case in patch 6f74bc8b6e8d related to the CWR flag. In that case, there are 5 packets with CWR values of 0, 1, 1, 0, and 0, and packets are flushed after the 1st, 3rd, and 5th packets. The gro.py uses this case in tools/testing/selftests/net/lib/gro.c to verify CWR behavior. But indeed, that does not cover whether SKB_GSO_TCP_ECN or SKB_GSO_TCP_ACCECN shall be set during the GRO. So, a test might be added to verify the SKB_GSO_TCP_ECN or SKB_GSO_TCP_ACCECN flags (if there is another suggested way, please let me know)? > The comment as stated seems to be misleading - there's nothing wrong with > using the flag if the device follows the RFC3168 semantics correctly. > > And of course, adding a comment and hoping people will find it is much weaker > than adding tests. > > Again, maybe I'm missing what _actually_ doesn't work here. Before adding an extra test, we need to clarify the definition and usages of these flags. At the TX path, in tcp_gso_segment() of net/ipv4/tcp_offload.c, the SKB_GSO_TCP_ACCECN flag is used to preserve the CWR flags for AccECN flows. Otherwise, when without SKB_GSO_TCP_ACCECN (RFC3168 ECN or Non-ECN flows), cwr will be cleared from the following packets. For the RX path, unfortunately I do not find a clear rule of when SKB_GSO_TCP_ECN shall be set except in include/linux/skbuff.h. Plus, the device usually does not track packets belonging to RFC3168 ECN or ACCECN flows. So, my previous thought is to always use SKB_GSO_TCP_ACCECN in the RX path to avoid any potential CWR bleaching. And FYI we will further include the related changes on the virtio-net after we get an approval in the virtio-spec.
