Re: [net-next 01/12] net/mlx5e: Add UDP GSO support

[Boris Pismenny <borisp@mellanox.com>]

On 7/2/2018 6:32 PM, Alexander Duyck wrote:
> 
> 
> On Mon, Jul 2, 2018 at 7:46 AM Willem de Bruijn 
> <willemdebruijn.kernel@gmail.com 
> <mailto:willemdebruijn.kernel@gmail.com>> wrote:
> 
>     On Mon, Jul 2, 2018 at 9:34 AM Willem de Bruijn
>     <willemdebruijn.kernel@gmail.com
>     <mailto:willemdebruijn.kernel@gmail.com>> wrote:
>      >
>      > On Mon, Jul 2, 2018 at 1:30 AM Boris Pismenny
>     <borisp@mellanox.com <mailto:borisp@mellanox.com>> wrote:
>      > >
>      > >
>      > >
>      > > On 7/2/2018 4:45 AM, Willem de Bruijn wrote:
>      > > >>> I've noticed that we could get cleaner code in our driver
>     if we remove
>      > > >>> these two lines from net/ipv4/udp_offload.c:
>      > > >>> if (skb_is_gso(segs))
>      > > >>>                mss *= skb_shinfo(segs)->gso_segs;
>      > > >>>
>      > > >>> I think that this is correct in case of GSO_PARTIAL
>     segmentation for the
>      > > >>> following reasons:
>      > > >>> 1. After this change the UDP payload field is consistent
>     with the IP
>      > > >>> header payload length field. Currently, IPv4 length is 1500
>     and UDP
>      > > >>> total length is the full unsegmented length.
>      > > >
>      > > > How does this simplify the driver? Does it currently have to
>      > > > change the udph->length field to the mss on the wire, because the
>      > > > device only splits + replicates the headers + computes the csum?
>      > >
>      > > Yes, this is the code I have at the moment.
>      > >
>      > > The device's limitation is more subtle than this. It could
>     adjust the
>      > > length, but then the checksum would be wrong.
>      >
>      > I see. We do have to keep in mind other devices. Alexander's ixgbe
>      > RFC patch does not have this logic, so that device must update the
>      > field directly.
>      >
>      > https://patchwork.ozlabs.org/patch/908396/
> 
>     To be clear, I think it's fine to remove these two lines if it does
>     not cause
>     problems for the ixgbe. It's quite possible that that device sets
>     the udp
>     length field unconditionally, ignoring the previous value. In which
>     case both
>     devices will work after this change without additional driver logic.
> 
> 
> I would prefer we didn’t modify this. Otherwise we cannot cancel out the 
> length from the partial checksum when the time comes. The ixgbe code was 
> making use of it if I recall.
> 

AFAIU, the ixgbe patch doesn't use this. Instead the length is obtained 
by the following code for both TCP and UDP segmentation:
paylen = skb->len - l4_offset;

Could you please check to see if this is actually required?