netdev - Re: [PATCH net-next v3 2/5] net: Disable GRO_HW when generic XDP is installed on a device.

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Date:   Sat, 9 Dec 2017 13:40:31 -0800
From:   Michael Chan <michael.chan@...adcom.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>,
        Andrew Gospodarek <andrew.gospodarek@...adcom.com>,
        Ariel Elior <Ariel.Elior@...ium.com>,
        everest-linux-l2@...ium.com
Subject: Re: [PATCH net-next v3 2/5] net: Disable GRO_HW when generic XDP is
 installed on a device.

On Sat, Dec 9, 2017 at 10:56 AM, Alexander Duyck
<alexander.duyck@...il.com> wrote:
> On Fri, Dec 8, 2017 at 10:27 PM, Michael Chan <michael.chan@...adcom.com> wrote:
>> Hardware should not aggregate any packets when generic XDP is installed.
>>
>> Cc: Ariel Elior <Ariel.Elior@...ium.com>
>> Cc: everest-linux-l2@...ium.com
>> Signed-off-by: Michael Chan <michael.chan@...adcom.com>
>> ---
>>  net/core/dev.c | 24 ++++++++++++++++++++++++
>>  1 file changed, 24 insertions(+)
>>
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index 6ebd0e7..ec08ace 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -1542,6 +1542,29 @@ void dev_disable_lro(struct net_device *dev)
>>  }
>>  EXPORT_SYMBOL(dev_disable_lro);
>>
>> +/**
>> + *     dev_disable_gro_hw - disable HW Generic Receive Offload on a device
>> + *     @dev: device
>> + *
>> + *     Disable HW Generic Receive Offload (GRO_HW) on a net device.  Must be
>> + *     called under RTNL.  This is needed if Generic XDP is installed on
>> + *     the device.
>> + */
>> +static void dev_disable_gro_hw(struct net_device *dev)
>> +{
>> +       struct net_device *lower_dev;
>> +       struct list_head *iter;
>> +
>> +       dev->wanted_features &= ~NETIF_F_GRO_HW;
>> +       netdev_update_features(dev);
>> +
>> +       if (unlikely(dev->features & NETIF_F_GRO_HW))
>> +               netdev_WARN(dev, "failed to disable GRO_HW!\n");
>> +
>> +       netdev_for_each_lower_dev(dev, lower_dev, iter)
>> +               dev_disable_gro_hw(lower_dev);
>
> I think these two lines are redundant in dev_disable_lro, since
> netdev_update_features should propagate the disable to all of the
> lower devices.

Right.  But for GRO_HW, there is no automatic propagation.

> Also this doesn't prevent the lower devices from
> re-enabling gro_hw.

Right.  You can re-enable LRO on the upper device as well.