netdev - RE: [net] e1000: Small packets may get corrupted during padding by HW

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Message-ID: <061C8A8601E8EE4CA8D8FD6990CEA89130DC3631@ORSMSX102.amr.corp.intel.com>
Date:	Mon, 17 Sep 2012 07:33:12 +0000
From:	"Dave, Tushar N" <tushar.n.dave@...el.com>
To:	"Fastabend, John R" <john.r.fastabend@...el.com>
CC:	Michal Miroslaw <mirqus@...il.com>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"gospo@...hat.com" <gospo@...hat.com>,
	"sassmann@...hat.com" <sassmann@...hat.com>
Subject: RE: [net] e1000: Small packets may get corrupted during padding by
 HW

>-----Original Message-----
>From: netdev-owner@...r.kernel.org [mailto:netdev-owner@...r.kernel.org]
>On Behalf Of John Fastabend
>Sent: Saturday, September 15, 2012 6:49 PM
>To: Dave, Tushar N
>Cc: Michal Miroslaw; Kirsher, Jeffrey T; davem@...emloft.net;
>netdev@...r.kernel.org; gospo@...hat.com; sassmann@...hat.com
>Subject: Re: [net] e1000: Small packets may get corrupted during padding
>by HW
>
>On 9/15/2012 6:25 PM, Dave, Tushar N wrote:
>>> -----Original Message-----
>>> From: Michał Mirosław [mailto:mirqus@...il.com]
>>> Sent: Saturday, September 15, 2012 1:45 PM
>>> To: Kirsher, Jeffrey T
>>> Cc: davem@...emloft.net; Dave, Tushar N; netdev@...r.kernel.org;
>>> gospo@...hat.com; sassmann@...hat.com
>>> Subject: Re: [net] e1000: Small packets may get corrupted during
>>> padding by HW
>>>
>>> 2012/9/15 Jeff Kirsher <jeffrey.t.kirsher@...el.com>:
>>>> From: Tushar Dave <tushar.n.dave@...el.com>
>>>>
>>>> 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 <tushar.n.dave@...el.com>
>>>> Tested-by: Aaron Brown <aaron.f.brown@...el.com>
>>>> Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@...el.com>
>>>> ---
>>>>   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?

No because it is quite normal to have packet < ETH_ZLEN. e.g. ARP packets.
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