netdev - Re: [PATCH 4/4] tun: indicate support for USO feature

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Date:   Wed, 12 May 2021 09:33:14 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Yuri Benditovich <yuri.benditovich@...nix.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        Yan Vugenfirer <yan@...nix.com>
Subject: Re: [PATCH 4/4] tun: indicate support for USO feature


在 2021/5/11 下午4:33, Yuri Benditovich 写道:
> On Tue, May 11, 2021 at 9:50 AM Jason Wang <jasowang@...hat.com> wrote:
>>
>> 在 2021/5/11 下午12:42, Yuri Benditovich 写道:
>>> Signed-off-by: Yuri Benditovich <yuri.benditovich@...nix.com>
>>> ---
>>>    drivers/net/tun.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>>> index 84f832806313..a35054f9d941 100644
>>> --- a/drivers/net/tun.c
>>> +++ b/drivers/net/tun.c
>>> @@ -2812,7 +2812,7 @@ static int set_offload(struct tun_struct *tun, unsigned long arg)
>>>                        arg &= ~(TUN_F_TSO4|TUN_F_TSO6);
>>>                }
>>>
>>> -             arg &= ~TUN_F_UFO;
>>> +             arg &= ~(TUN_F_UFO|TUN_F_USO);
>>
>> It looks to me kernel doesn't use "USO", so TUN_F_UDP_GSO_L4 is a better
>> name for this
> No problem, I can change it in v2
>
>   and I guess we should toggle NETIF_F_UDP_GSO_l4 here?
>
> No, we do not, because this indicates only the fact that the guest can
> send large UDP packets and have them splitted to UDP segments.


Actually the reverse. The set_offload() controls the tuntap TX path 
(guest RX path).

When VIRTIO_NET_F_GUEST_XXX was not negotiated, the corresponding netdev 
features needs to be disabled. When host tries to send those packets to 
guest, it needs to do software segmentation.

See virtio_net_apply_guest_offloads().

There's currently no way (or not need) to prevent tuntap from receiving 
GSO packets.

Thanks


>
>> And how about macvtap?
> We will check how to do that for macvtap. We will send a separate
> patch for macvtap or ask for advice.
>
>> Thanks
>>
>>
>>>        }
>>>
>>>        /* This gives the user a way to test for new features in future by