netdev - Re: [PATCH net] net/mlx4_en: Force CHECKSUM

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Date:   Thu, 31 Jan 2019 19:04:28 +0000
From:   Saeed Mahameed <saeedm@...lanox.com>
To:     Tariq Toukan <tariqt@...lanox.com>,
        "edumazet@...gle.com" <edumazet@...gle.com>
CC:     Eran Ben Elisha <eranbe@...lanox.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net] net/mlx4_en: Force CHECKSUM_NONE for short ethernet
 frames

On Thu, 2019-01-31 at 07:02 -0800, Eric Dumazet wrote:
> On Thu, Jan 31, 2019 at 5:04 AM Tariq Toukan <tariqt@...lanox.com>
> wrote:
> > From: Saeed Mahameed <saeedm@...lanox.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 <edumazet@...gle.com>
> > Signed-off-by: Saeed Mahameed <saeedm@...lanox.com>
> > Signed-off-by: Tariq Toukan <tariqt@...lanox.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.. 


> 
> > +        */
> > +       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
> >