On Thu, Jan 31, 2019 at 11:04 AM Saeed Mahameed <sae...@mellanox.com> wrote:
>
> On Thu, 2019-01-31 at 07:02 -0800, Eric Dumazet wrote:
> > On Thu, Jan 31, 2019 at 5:04 AM Tariq Toukan <tar...@mellanox.com>
> > wrote:
> > > From: Saeed Mahameed <sae...@mellanox.com>
> > >
> > > When an ethernet frame is padded to meet the minimum ethernet frame
> > > size, the padding octets are not covered by the hardware checksum.
> > > Fortunately the padding octets are usually zero's, which don't
> > > affect
> > > checksum. However, it is not guaranteed. For example, switches
> > > might
> > > choose to make other use of these octets.
> > > This repeatedly causes kernel hardware checksum fault.
> > >
> > > Prior to the cited commit below, skb checksum was forced to be
> > > CHECKSUM_NONE when padding is detected. After it, we need to keep
> > > skb->csum updated. However, fixing up CHECKSUM_COMPLETE requires to
> > > verify and parse IP headers, it does not worth the effort as the
> > > packets
> > > are so small that CHECKSUM_COMPLETE has no significant advantage.
> > >
> > > Fixes: 88078d98d1bb ("net: pskb_trim_rcsum() and CHECKSUM_COMPLETE
> > > are friends")
> > > Cc: Eric Dumazet <eduma...@google.com>
> > > Signed-off-by: Saeed Mahameed <sae...@mellanox.com>
> > > Signed-off-by: Tariq Toukan <tar...@mellanox.com>
> > > ---
> > > drivers/net/ethernet/mellanox/mlx4/en_rx.c | 19 +++++++++++++++++-
> > > -
> > > 1 file changed, 17 insertions(+), 2 deletions(-)
> > >
> > > Hi Dave,
> > > Please queue for -stable >= v4.18.
> > >
> > > Thanks,
> > > Tariq
> > >
> > > diff --git a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> > > b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> > > index 9a0881cb7f51..fc8a11444e1a 100644
> > > --- a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> > > +++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> > > @@ -617,6 +617,8 @@ static int get_fixed_ipv6_csum(__wsum
> > > hw_checksum, struct sk_buff *skb,
> > > }
> > > #endif
> > >
> > > +#define short_frame(size) ((size) <= ETH_ZLEN + ETH_FCS_LEN)
> > > +
> > > /* We reach this function only after checking that any of
> > > * the (IPv4 | IPv6) bits are set in cqe->status.
> > > */
> > > @@ -624,9 +626,22 @@ static int check_csum(struct mlx4_cqe *cqe,
> > > struct sk_buff *skb, void *va,
> > > netdev_features_t dev_features)
> > > {
> > > __wsum hw_checksum = 0;
> > > + void *hdr;
> > > +
> > > + /* CQE csum doesn't cover padding octets in short ethernet
> > > + * frames. And the pad field is appended prior to
> > > calculating
> > > + * and appending the FCS field.
> > > + *
> > > + * Detecting these padded frames requires to verify and
> > > parse
> > > + * IP headers, so we simply force all those small frames to
> > > be
> > > + * CHECKSUM_NONE even if they are not padded.
> > > + * TODO: better if we report CHECKSUM_UNNECESSARY but this
> > > + * demands some refactroing.
> >
> > This TODO: looks bogus to me.
> >
> > mlx4 driver first tries to use CHECKSUM_UNNECESSARY.
> >
> > if ((cqe->status & cpu_to_be16(MLX4_CQE_STATUS_TCP |
> > MLX4_CQE_STATUS_UDP
> > )) &&
> > (cqe->status & cpu_to_be16(MLX4_CQE_STATUS_IPOK)) &&
> > cqe->checksum == cpu_to_be16(0xffff)) {
> > ...
> > ip_summed = CHECKSUM_UNNECESSARY;
> > ...
> > }
> >
> > Then if this attempt failed, it tries to use CHECKSUM_COMPLETE (and
> > calls this check_sum() function)
> >
> > So there is no refactoring to be done for mlx4 : short
> > IPv[46]+{UDP|TCP} frames get CHECKSUM_UNNECESSARY already
>
> not exactly, considering mlx4 weird condition to decide to do
> CHECKSUM_UNNECESSARY:
>
> if ((cqe csum fields are valid) && cqe->checksum ==
> cpu_to_be16(0xffff))
> { do CHECKSUM_UNNECESSARY }
> else { do CHECKSUM_COMPLETE }
>
> CHECKSUM_UNNECESSARY will be skipped if csum complete field is valid
> i.e (cqe->checksum != 0xffff)
>
> but if checksum complete is to be skipped due to short_frame we want to
> go back to CHECKSUM_UNNECESSARY, this is the refactoring i am talking
> about :).
>
>
> I hope now it is clear..
Not really, since in case of short frame, cqe->checksum will be
0xffff, since mlx4 does not include the padding bytes in the checksum
in the first place.
(For native IPv4/IPV6 TCP/UDP frames that is)
>
>
> >
> > > + */
> > > + if (short_frame(skb->len))
> > > + return -EINVAL;
> > >
> > > - void *hdr = (u8 *)va + sizeof(struct ethhdr);
> > > -
> > > + hdr = (u8 *)va + sizeof(struct ethhdr);
> > > hw_checksum = csum_unfold((__force __sum16)cqe->checksum);
> > >
> > > if (cqe->vlan_my_qpn &
> > > cpu_to_be32(MLX4_CQE_CVLAN_PRESENT_MASK) &&
> > > --
> > > 1.8.3.1
> > >
