linux-kernel - Re: [PATCH net] virtio-net: unbreak cusmed packet for small buffer XDP

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Message-ID: <69907228-fc1b-175f-f6cd-7ac332e318be@redhat.com>
Date:   Wed, 28 Jun 2017 10:45:18 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] virtio-net: unbreak cusmed packet for small buffer
 XDP



On 2017年06月28日 10:17, Michael S. Tsirkin wrote:
> On Wed, Jun 28, 2017 at 10:14:34AM +0800, Jason Wang wrote:
>>
>> On 2017年06月28日 10:02, Michael S. Tsirkin wrote:
>>> On Wed, Jun 28, 2017 at 09:54:03AM +0800, Jason Wang wrote:
>>>> We should allow csumed packet for small buffer, otherwise XDP_PASS
>>>> won't work correctly.
>>>>
>>>> Fixes commit bb91accf2733 ("virtio-net: XDP support for small buffers")
>>>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>>> The issue would be VIRTIO_NET_HDR_F_DATA_VALID might be set.
>>> What do you think?
>> I think it's safe. For XDP_PASS, it work like in the past.
> That's the part I don't get. With DATA_VALID csum in packet is wrong, XDP
> tools assume it's value.

DATA_VALID is CHECKSUM_UNCESSARY on the host, and according to the 
comment in skbuff.h


"
  *   The hardware you're dealing with doesn't calculate the full checksum
  *   (as in CHECKSUM_COMPLETE), but it does parse headers and verify 
checksums
  *   for specific protocols. For such packets it will set 
CHECKSUM_UNNECESSARY
  *   if their checksums are okay. skb->csum is still undefined in this case
  *   though. A driver or device must never modify the checksum field in the
  *   packet even if checksum is verified.
"

The csum is correct I believe?

Thanks

>
>> For XDP_TX, we
>> zero the vnet header.
> Again TX offload is disabled, so packets will go out with an invalid
> checksum.
>
>> For adjusting header, XDP prog should deal with csum.
>>
>> Thanks
> That part seems right.
>
>>>> ---
>>>> The patch is needed for -stable.
>>>> ---
>>>>    drivers/net/virtio_net.c | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>>> index 143d8a9..499fcc9 100644
>>>> --- a/drivers/net/virtio_net.c
>>>> +++ b/drivers/net/virtio_net.c
>>>> @@ -413,7 +413,7 @@ static struct sk_buff *receive_small(struct net_device *dev,
>>>>    		void *orig_data;
>>>>    		u32 act;
>>>> -		if (unlikely(hdr->hdr.gso_type || hdr->hdr.flags))
>>>> +		if (unlikely(hdr->hdr.gso_type))
>>>>    			goto err_xdp;
>>>>    		xdp.data_hard_start = buf + VIRTNET_RX_PAD + vi->hdr_len;
>>>> -- 
>>>> 2.7.4