On Wed, Jan 3, 2024 at 8:52 AM Martin KaFai Lau <martin.lau@xxxxxxxxx> wrote: > > On 1/2/24 10:11 AM, Stanislav Fomichev wrote: > > On 12/29, Menglong Dong wrote: > >> For now, we have to call some helpers when we need to update the csum, > >> such as bpf_l4_csum_replace, bpf_l3_csum_replace, etc. These helpers are > >> not inlined, which causes poor performance. > >> > >> In fact, we can define our own csum update functions in BPF program > >> instead of bpf_l3_csum_replace, which is totally inlined and efficient. > >> However, we can't do this for bpf_l4_csum_replace for now, as we can't > >> update skb->csum, which can cause skb->csum invalid in the rx path with > >> CHECKSUM_COMPLETE mode. > >> > >> What's more, we can't use the direct data access and have to use > >> skb_store_bytes() with the BPF_F_RECOMPUTE_CSUM flag in some case, such > >> as modifing the vni in the vxlan header and the underlay udp header has > >> no checksum. > > There is bpf_csum_update(), does it work? > A helper call should be acceptable comparing with the csum calculation itself. Yeah, this helper works in this case! Now we miss the last piece for the tx path: ip_summed. We need to know if it is CHECKSUM_PARTIAL to decide if we should update the csum in the packet. In the tx path, the csum in the L4 is the pseudo header only if skb->ip_summed is CHECKSUM_PARTIAL. Maybe we can introduce a lightweight kfunc to get its value? Such as bpf_skb_csum_mode(). As we need only call it once, there shouldn't be overhead on it. Thanks! Menglong Dong > > >> > >> In the first patch, we make skb->csum readable and writable, and we make > >> skb->ip_summed readable. For now, for tc only. With these 2 fields, we > >> don't need to call bpf helpers for csum update any more. > >> > >> In the second patch, we add some testcases for the read/write testing for > >> skb->csum and skb->ip_summed. > >> > >> If this series is acceptable, we can define the inlined functions for csum > >> update in libbpf in the next step. > > > > One downside of exposing those as __sk_buff fields is that all this > > skb internal csum stuff now becomes a UAPI. And I'm not sure we want > > +1. Please no new __sk_buff extension and no new conversion in > bpf_convert_ctx_access(). > > > that :-) Should we add a lightweight kfunc to reset the fields instead? > > Or will it still have an unacceptable overhead? >
