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Subject: Re: [PATCH 4/4] tun: indicate support for USO feature
To:     Yuri Benditovich <yuri.benditovich@daynix.com>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        Network Development <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        virtualization@lists.linux-foundation.org,
        Yan Vugenfirer <yan@daynix.com>
References: <20210511044253.469034-1-yuri.benditovich@daynix.com>
 <20210511044253.469034-5-yuri.benditovich@daynix.com>
 <eb8c4984-f0cc-74ee-537f-fc60deaaaa73@redhat.com>
 <CAOEp5OdrCDPx4ijLcEOm=Wxma6hc=nyqw4Xm6bggBxvgtR0tbg@mail.gmail.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <89759261-3a72-df6c-7a81-b7a48abfad44@redhat.com>
Date:   Wed, 12 May 2021 09:33:14 +0800
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在 2021/5/11 下午4:33, Yuri Benditovich 写道:
> On Tue, May 11, 2021 at 9:50 AM Jason Wang <jasowang@redhat.com> wrote:
>>
>> 在 2021/5/11 下午12:42, Yuri Benditovich 写道:
>>> Signed-off-by: Yuri Benditovich <yuri.benditovich@daynix.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