Date:   Mon, 4 Dec 2017 08:47:41 -0800
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Michael Chan <michael.chan@...adcom.com>
Cc:     David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>,
        Ariel Elior <Ariel.Elior@...ium.com>,
        everest-linux-l2@...ium.com
Subject: Re: [PATCH net-next 1/4] net: Introduce NETIF_F_GRO_HW

On Mon, Dec 4, 2017 at 3:12 AM, Michael Chan <michael.chan@...adcom.com> wrote:
> Introduce NETIF_F_GRO_HW feature flag for NICs that support hardware
> GRO.  With this flag, we can now independently turn on or off hardware
> GRO when GRO is on.  Hardware GRO guarantees that packets can be
> re-segmented by TSO/GSO to reconstruct the original packet stream.
>
> Cc: Ariel Elior <Ariel.Elior@...ium.com>
> Cc: everest-linux-l2@...ium.com
> Signed-off-by: Michael Chan <michael.chan@...adcom.com>

Do we really need yet another feature bit for this? We already have
LRO and GRO and now we have to add something that isn't quite either
one?

I think I would rather have something like a netdev private flag that
says LRO assembled frames are routable and just have this all run over
the LRO flag with a test for the private flag to avoid triggering the
LRO disable in the case of the flag being present. Really this is just
a clean LRO implementation anyway so maybe we should just go that
route where LRO is the hardware offload and GRO is the generic
software implementation of that offload. That way when GRO gets some
new feature that your hardware doesn't support we don't have to argue
about the differences since LRO is meant to be a limited
implementation anyway due to the nature of it being in hardware.

To me it just seems like this is an attempt to use the GRO name as a
marketing term and I really don't like the feel of it.

> ---
>  Documentation/networking/netdev-features.txt |  7 +++++++
>  include/linux/netdev_features.h              |  5 ++++-
>  net/core/dev.c                               | 13 +++++++++++++
>  net/core/ethtool.c                           |  1 +
>  4 files changed, 25 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/networking/netdev-features.txt b/Documentation/networking/netdev-features.txt
> index 7413eb0..d76d332 100644
> --- a/Documentation/networking/netdev-features.txt
> +++ b/Documentation/networking/netdev-features.txt
> @@ -163,3 +163,10 @@ This requests that the NIC receive all possible frames, including errored
>  frames (such as bad FCS, etc).  This can be helpful when sniffing a link with
>  bad packets on it.  Some NICs may receive more packets if also put into normal
>  PROMISC mode.
> +
> +*  rx-gro-hw
> +
> +This requests that the NIC enables Hardware GRO (generic receive offload).
> +Hardware GRO is basically the exact reverse of TSO, and is generally
> +stricter than Hardware LRO.  A packet stream merged by Hardware GRO must
> +be re-segmentable by GSO or TSO back to the exact packet stream.
> diff --git a/include/linux/netdev_features.h b/include/linux/netdev_features.h
> index dc8b489..d18ef6f 100644
> --- a/include/linux/netdev_features.h
> +++ b/include/linux/netdev_features.h
> @@ -77,6 +77,8 @@ enum {
>         NETIF_F_HW_ESP_TX_CSUM_BIT,     /* ESP with TX checksum offload */
>         NETIF_F_RX_UDP_TUNNEL_PORT_BIT, /* Offload of RX port for UDP tunnels */
>
> +       NETIF_F_GRO_HW_BIT,             /* Hardware Generic receive offload */
> +
>         /*
>          * Add your fresh new feature above and remember to update
>          * netdev_features_strings[] in net/core/ethtool.c and maybe
> @@ -96,6 +98,7 @@ enum {
>  #define NETIF_F_FRAGLIST       __NETIF_F(FRAGLIST)
>  #define NETIF_F_FSO            __NETIF_F(FSO)
>  #define NETIF_F_GRO            __NETIF_F(GRO)
> +#define NETIF_F_GRO_HW         __NETIF_F(GRO_HW)
>  #define NETIF_F_GSO            __NETIF_F(GSO)
>  #define NETIF_F_GSO_ROBUST     __NETIF_F(GSO_ROBUST)
>  #define NETIF_F_HIGHDMA                __NETIF_F(HIGHDMA)
> @@ -193,7 +196,7 @@ enum {
>   * If upper/master device has these features disabled, they must be disabled
>   * on all lower/slave devices as well.
>   */
> -#define NETIF_F_UPPER_DISABLES NETIF_F_LRO
> +#define NETIF_F_UPPER_DISABLES (NETIF_F_LRO | NETIF_F_GRO_HW)
>
>  /* changeable features with no special hardware requirements */
>  #define NETIF_F_SOFT_FEATURES  (NETIF_F_GSO | NETIF_F_GRO)
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 30b5fe3..09c2ad0 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -7392,6 +7392,19 @@ static netdev_features_t netdev_fix_features(struct net_device *dev,
>                 features &= ~dev->gso_partial_features;
>         }
>
> +       if (features & NETIF_F_GRO_HW) {
> +               /* Hardware GRO depends on GRO. */
> +               if (!(features & NETIF_F_GRO)) {
> +                       netdev_dbg(dev, "Dropping NETIF_F_GSO_HW since no GRO feature.\n");
> +                       features &= ~NETIF_F_GRO_HW;
> +               }
> +               /* Hardware GRO and LRO are mutually exclusive. */
> +               if (features & NETIF_F_LRO) {
> +                       netdev_dbg(dev, "Dropping NETIF_F_LRO since GRO_HW is set.\n");
> +                       features &= ~NETIF_F_LRO;
> +               }
> +       }
> +
>         return features;
>  }
>
> diff --git a/net/core/ethtool.c b/net/core/ethtool.c
> index f8fcf45..50a7920 100644
> --- a/net/core/ethtool.c
> +++ b/net/core/ethtool.c
> @@ -73,6 +73,7 @@ int ethtool_op_get_ts_info(struct net_device *dev, struct ethtool_ts_info *info)
>         [NETIF_F_LLTX_BIT] =             "tx-lockless",
>         [NETIF_F_NETNS_LOCAL_BIT] =      "netns-local",
>         [NETIF_F_GRO_BIT] =              "rx-gro",
> +       [NETIF_F_GRO_HW_BIT] =           "rx-gro-hw",
>         [NETIF_F_LRO_BIT] =              "rx-lro",
>
>         [NETIF_F_TSO_BIT] =              "tx-tcp-segmentation",
> --
> 1.8.3.1
>

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