Subject: Re: [net] e1000: Small packets may get corrupted during padding by HW

From: John Fastabend <[email-masked]>
Date: Sat, 15 Sep 2012 18:48:33 -0700

On 9/15/2012 6:25 PM, Dave, Tushar N wrote:
>> -----Original Message-----
>> From: Michał Mirosław [mailto:[email-masked]]
>> Sent: Saturday, September 15, 2012 1:45 PM
>> To: Kirsher, Jeffrey T
>> Cc: [email-masked]; Dave, Tushar N; [email-masked];
>> [email-masked]; [email-masked]
>> Subject: Re: [net] e1000: Small packets may get corrupted during padding
>> by HW
>>
>> 2012/9/15 Jeff Kirsher <[email-masked]>:
>>> From: Tushar Dave <[email-masked]>
>>>
>>> On PCI/PCI-X HW, if packet size is less than ETH_ZLEN, packets may get
>>> corrupted during padding by HW.
>>> To WA this issue, pad all small packets manually.
>>>
>>> Signed-off-by: Tushar Dave <[email-masked]>
>>> Tested-by: Aaron Brown <[email-masked]>
>>> Signed-off-by: Jeff Kirsher <[email-masked]>
>>> ---
>>>   drivers/net/ethernet/intel/e1000/e1000_main.c | 11 +++++++++++
>>>   1 file changed, 11 insertions(+)
>>>
>>> diff --git a/drivers/net/ethernet/intel/e1000/e1000_main.c
>>> b/drivers/net/ethernet/intel/e1000/e1000_main.c
>>> index 3bfbb8d..bde337e 100644
>>> --- a/drivers/net/ethernet/intel/e1000/e1000_main.c
>>> +++ b/drivers/net/ethernet/intel/e1000/e1000_main.c
>>> @@ -3149,6 +3149,17 @@ static netdev_tx_t e1000_xmit_frame(struct
>> sk_buff *skb,
>>>                  return NETDEV_TX_OK;
>>>          }
>>>
>>> +       /* On PCI/PCI-X HW, if packet size is less than ETH_ZLEN,
>>> +        * packets may get corrupted during padding by HW.
>>> +        * To WA this issue, pad all small packets manually.
>>> +        */
>>> +       if (skb->len < ETH_ZLEN) {
>>> +               if (skb_pad(skb, ETH_ZLEN - skb->len))
>>> +                       return NETDEV_TX_OK;
>>> +               skb->len = ETH_ZLEN;
>>> +               skb_set_tail_pointer(skb, ETH_ZLEN);
>>> +       }
>>> +
>>
>> Isn't there a skb_padto() that does just this?
>
> Skb_padto calls skb_pad(). Calling skb_pad directly saves some cycles.
>

How/where?

static inline int skb_padto(struct sk_buff *skb, unsigned int len)
{
         unsigned int size = skb->len;
         if (likely(size >= len))
                 return 0;
         return skb_pad(skb, len - size);
}

Also wouldn't you want an unlikely() in your patch?

.John