netdev - Re: [PATCH net] netdevice: Prefer NETIF_F_HW

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Message-ID: <CAKgT0Ue67jdAcFeyXqv4Nr8+A-oKNBk7+xV_WmcfWdrNRgRGTQ@mail.gmail.com>
Date:   Thu, 20 Apr 2017 11:36:26 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Vladislav Yasevich <vyasevich@...il.com>
Cc:     Netdev <netdev@...r.kernel.org>, Tom Herbert <tom@...bertland.com>,
        Michal Kubecek <mkubecek@...e.cz>,
        Vladislav Yasevich <vyasevic@...hat.com>
Subject: Re: [PATCH net] netdevice: Prefer NETIF_F_HW_CSUM when intersecting features

On Wed, Apr 19, 2017 at 6:12 PM, Vladislav Yasevich <vyasevich@...il.com> wrote:
> While hardware device use either NETIF_F_(IP|IPV6)_CSUM or
> NETIF_F_HW_CSUM, all of the software devices use HW_CSUM.
> This results in an interesting situation when the software
> device is configured on top of hw device using (IP|IPV6)_CSUM.
> In this situation, the user can't turn off checksum offloading
> features on the software device.
>
> This patch resolves that by prefering the NETIF_F_HW_CSUM setting
> when computing a feature intersect.
>
> Signed-off-by: Vladislav Yasevich <vyasevic@...hat.com>
> ---
>  include/linux/netdevice.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 97456b25..3d811c1 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -4019,9 +4019,9 @@ static inline netdev_features_t netdev_intersect_features(netdev_features_t f1,
>  {
>         if ((f1 ^ f2) & NETIF_F_HW_CSUM) {
>                 if (f1 & NETIF_F_HW_CSUM)
> -                       f1 |= (NETIF_F_IP_CSUM|NETIF_F_IPV6_CSUM);
> +                       f2 |= NETIF_F_HW_CSUM;
>                 else
> -                       f2 |= (NETIF_F_IP_CSUM|NETIF_F_IPV6_CSUM);
> +                       f1 |= NETIF_F_HW_CSUM;
>         }
>
>         return f1 & f2;
> --
> 2.7.4
>

This doesn't seem right to me. I can see promoting NETIF_F_HW_CSUM to
also include IP_CSUM and IPV6_CSUM but all this seems to do is force
NETIF_F_HW_CSUM on always if either side has it enabled. That doesn't
seem like an intersection at all to me.

What this code is supposed to do is make it so that if we are testing
for NETIF_F_IP_CSUM and NETIF_F_HW_CSUM is set it will return true.
The code as it is now will always return true if we test for
NETIF_F_HW_CSUM since it will just set it on the other side for us.

- Alex