netdev - Re: [PATCH net] net/mlx5: Correctly set gso

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Message-ID: <CADg4-L86-6Bn8trYyd-+KdQ4NMmYKa5z+zmo32StYYmBBRpcdw@mail.gmail.com>
Date: Mon, 14 Jul 2025 09:49:18 -0700
From: Christoph Paasch <cpaasch@...nai.com>
To: Tariq Toukan <ttoukan.linux@...il.com>
Cc: Saeed Mahameed <saeedm@...dia.com>, Tariq Toukan <tariqt@...dia.com>, Mark Bloch <mbloch@...dia.com>, 
	Leon Romanovsky <leon@...nel.org>, Andrew Lunn <andrew+netdev@...n.ch>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Amir Vadai <amirv@...lanox.com>, 
	netdev@...r.kernel.org, linux-rdma@...r.kernel.org
Subject: Re: [PATCH net] net/mlx5: Correctly set gso_size when LRO is used

On Sun, Jul 13, 2025 at 11:49 PM Tariq Toukan <ttoukan.linux@...il.com> wrote:
>
>
>
> On 10/07/2025 21:26, christoph.paasch@...il.com wrote:
> > From: Christoph Paasch <cpaasch@...nai.com>
> >
> > gso_size is expected by the networking stack to be the size of the
> > payload (thus, not including ethernet/IP/TCP-headers). However, cqe_bcnt
> > is the full sized frame (including the headers). Dividing cqe_bcnt by
> > lro_num_seg will then give incorrect results.
> >
> > For example, running a bpftrace higher up in the TCP-stack
> > (tcp_event_data_recv), we commonly have gso_size set to 1450 or 1451 even
> > though in reality the payload was only 1448 bytes.
> >
> > So, we need to discount the protocol headers from cqe_bcnt so we can
> > actually divide the payload by lro_num_seg to get the real gso_size.
> >
> > Fixes: e586b3b0baee ("net/mlx5: Ethernet Datapath files")
> > Signed-off-by: Christoph Paasch <cpaasch@...nai.com>
> > ---
> >   .../net/ethernet/mellanox/mlx5/core/en_rx.c   | 20 +++++++++++++++----
> >   1 file changed, 16 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> > index 84b1ab8233b8..e23bb80b0e0d 100644
> > --- a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> > +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> > @@ -1154,12 +1154,14 @@ static void mlx5e_lro_update_tcp_hdr(struct mlx5_cqe64 *cqe, struct tcphdr *tcp)
> >       }
> >   }
> >
> > -static void mlx5e_lro_update_hdr(struct sk_buff *skb, struct mlx5_cqe64 *cqe,
> > -                              u32 cqe_bcnt)
> > +static unsigned int mlx5e_lro_update_hdr(struct sk_buff *skb,
> > +                                      struct mlx5_cqe64 *cqe,
> > +                                      u32 cqe_bcnt)
> >   {
> >       struct ethhdr   *eth = (struct ethhdr *)(skb->data);
> >       struct tcphdr   *tcp;
> >       int network_depth = 0;
> > +     unsigned int hdrlen;
> >       __wsum check;
> >       __be16 proto;
> >       u16 tot_len;
> > @@ -1169,11 +1171,14 @@ static void mlx5e_lro_update_hdr(struct sk_buff *skb, struct mlx5_cqe64 *cqe,
> >
> >       tot_len = cqe_bcnt - network_depth;
> >       ip_p = skb->data + network_depth;
> > +     hdrlen = network_depth;
> >
> >       if (proto == htons(ETH_P_IP)) {
> >               struct iphdr *ipv4 = ip_p;
> >
> >               tcp = ip_p + sizeof(struct iphdr);
> > +             hdrlen += sizeof(struct iphdr);
> > +
> >               skb_shinfo(skb)->gso_type = SKB_GSO_TCPV4;
> >
> >               ipv4->ttl               = cqe->lro.min_ttl;
> > @@ -1193,6 +1198,8 @@ static void mlx5e_lro_update_hdr(struct sk_buff *skb, struct mlx5_cqe64 *cqe,
> >               struct ipv6hdr *ipv6 = ip_p;
> >
> >               tcp = ip_p + sizeof(struct ipv6hdr);
> > +             hdrlen += sizeof(struct ipv6hdr);
> > +
> >               skb_shinfo(skb)->gso_type = SKB_GSO_TCPV6;
> >
> >               ipv6->hop_limit         = cqe->lro.min_ttl;
> > @@ -1205,6 +1212,10 @@ static void mlx5e_lro_update_hdr(struct sk_buff *skb, struct mlx5_cqe64 *cqe,
> >               tcp->check = tcp_v6_check(payload_len, &ipv6->saddr,
> >                                         &ipv6->daddr, check);
> >       }
> > +
> > +     hdrlen += tcp->doff * 4;
> > +
>
>
> Thanks for your patch!
>
> Calculations seem correct.
> Wouldn't it be simpler to just return the below?
>
> (void *)tcp + tcp->doff * 4 - skb->data

Absolutely! I can do that!


Christoph

>
> > +     return hdrlen;
> >   }
> >
> >   static void *mlx5e_shampo_get_packet_hd(struct mlx5e_rq *rq, u16 header_index)
> > @@ -1561,8 +1572,9 @@ static inline void mlx5e_build_rx_skb(struct mlx5_cqe64 *cqe,
> >               mlx5e_macsec_offload_handle_rx_skb(netdev, skb, cqe);
> >
> >       if (lro_num_seg > 1) {
> > -             mlx5e_lro_update_hdr(skb, cqe, cqe_bcnt);
> > -             skb_shinfo(skb)->gso_size = DIV_ROUND_UP(cqe_bcnt, lro_num_seg);
> > +             unsigned int hdrlen = mlx5e_lro_update_hdr(skb, cqe, cqe_bcnt);
> > +
> > +             skb_shinfo(skb)->gso_size = DIV_ROUND_UP(cqe_bcnt - hdrlen, lro_num_seg);
> >               /* Subtract one since we already counted this as one
> >                * "regular" packet in mlx5e_complete_rx_cqe()
> >                */
>
>