netdev - Re: [RFC][net-next-2.6 PATCH 2/4] net: 8021Q consolidate header

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Date:	Thu, 4 Nov 2010 11:26:48 -0700
From:	Jesse Gross <jesse@...ira.com>
To:	John Fastabend <john.r.fastabend@...el.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [RFC][net-next-2.6 PATCH 2/4] net: 8021Q consolidate header_ops routines

On Thu, Nov 4, 2010 at 6:43 AM, John Fastabend
<john.r.fastabend@...el.com> wrote:
> On 11/3/2010 5:47 PM, Jesse Gross wrote:
>> On Thu, Oct 21, 2010 at 3:10 PM, John Fastabend
>> <john.r.fastabend@...el.com> wrote:
>>> The only thing the 8021Q header ops routines are required
>>> for is the VLAN_FLAG_REORDER_HDR otherwise by the time
>>> the VLAN tag has been added the packet is already on
>>> its way down the stack. In this case using the Ethernet
>>> ops works OK.
>>>
>>> At present the VLAN_FLAG_REORDER_HDR flag does not work
>>> with vlan offloads. As I understand the flag the intent
>>> is to allow taps on the vlan device and possibly the
>>> QOS layer to see the vlan tag info.
>>>
>>> By inserting the tag in vlan_tci any taps or QOS policies
>>> should be able to retrieve the vlan info. This allows
>>> the flag to work the same in both the offload case and
>>> non-offloaded case. And allows us to use the underlying
>>> ethernet ops.
>>>
>>> Signed-off-by: John Fastabend <john.r.fastabend@...el.com>
>>
>> I noticed that you dropped this patch from your most recent series, so
>> I went back to take a look at it.  I realized that it probably works
>> inconsistently since header caching doesn't take into account
>> skb->vlan_tci, so whether you see the tag depends on the state of the
>> cache.
>>
>> It would be really good to have this type of code consolidation, both
>> for the sake of sanity and to eliminate the inconsistent behavior.  We
>> could do that by either not using header caching or making it work
>> with vlan offloading somehow.  However, I'm not sure that there's
>> really much point in that.  VLAN_FLAG_REORDER_HDR doesn't work with
>> cards that do vlan offloading, which is a pretty significant number of
>> them.  It similarly works inconsistently on the rx side.  So it's
>> broken most of the time and worse, the behavior changes depending on
>> the NIC (and now the ethtool setting).  Can we just eliminate it?
>
> Yes this is why I have dropped it for now. Also rebuild is broke as best I can tell. Although I doubt anyone would notice you would need to clear VLAN_FLAG_REORDER_HDR and be using one of the ARPHRD_{ROSE|AX25|NETROM}.
>
> The problem with caching the vlan header is the skb priority to vlan priority map. So we could cache the vid, sa, da, and protocols but I can not see anyway to cache the vlan priority. Also the cache would have to be flushed when the flag is toggled.

I agree, fixing this so that !VLAN_FLAG_REORDER_HDR works correctly in
all cases would be messy.

However, this has been broken for a long time and I don't know of
anyone complaining.  Since it is already a no-op in the accelerated
case, I would like to just drop the flag so we get consistent behavior
and less code.
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