Jesse Gross wrote:

On Fri, Oct 15, 2010 at 2:16 AM, Guillaume Gaudonville
[off-list ref] wrote:
  

quoted

Jesse Gross wrote:
    

quoted

On Thu, Sep 30, 2010 at 1:07 AM, Roger Luethi [off-list ref] wrote:

      

quoted

On Wed, 29 Sep 2010 10:44:26 -0700, Jesse Gross wrote:

        

quoted

On Wed, Sep 29, 2010 at 4:37 AM, Roger Luethi [off-list ref] wrote:

          

quoted

I noticed packets for unknown VLANs getting silently dropped even in
promiscuous mode (this is true only for the hardware accelerated path).
netif_nit_deliver was introduced specifically to prevent that, but the
function gets called only _after_ packets from unknown VLANs have been
dropped.

            

Some drivers are fixing this on a case by case basis by disabling
hardware accelerated VLAN stripping when in promiscuous mode, i.e.:

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=5f6c01819979afbfec7e0b15fe52371b8eed87e8

However, at this point it is more or less random which drivers do
this.  It would obviously be much better if it were consistent.

          

My understanding is this. Hardware VLAN tagging and stripping can always
be
enabled. The kernel passes 802.1Q information along with the stripped
header to libpcap which reassembles the original header where necessary.
Works for me.

        

Sorry, I misread your original post as saying that the VLAN header
gets dropped, rather than the entire packet.  I agree that this is how
it should work but not necessarily how it does work (again, depending
on the driver).  Here's the problem that I was talking about:

Most drivers have a snippet of code that looks something like this
(taken from ixgbe):

if (adapter->vlgrp && is_vlan && (tag & VLAN_VID_MASK))
       vlan_gro_receive(napi, adapter->vlgrp, tag, skb);
else
       napi_gro_receive(napi, skb);

At this point the VLAN has already been stripped in hardware.  If
there is no VLAN group configured on the device then we hit the second
case.  The VLAN header was removed from the SKB and the tag variable
is unused.  It is no longer possible for libpcap to reconstruct the
header because the information was thrown away (even the fact that
there was a VLAN tag at all).

There are a couple ways to fix this:

* Turn off VLAN stripping when in promiscuous mode (as done by the ixgbe
driver)

      

This is not totally true: if changing the MTU ixgbe_change_mtu will call:
ixgbe_reinit_locked--> ixgbe_up --> ixgbe_configure:
               --> ixgbe_set_rx_mode: flag IFF_PROMISC is tested
ixgbe_vlan_filter_enable is not called
               --> ixgbe_restore_vlan --> ixgbe_vlan_rx_register: flag
IFF_PROMISC is not tested ixgbe_vlan_filter_enable
                    will be called.

In fact it should happen each time we configure something which needs a
reset of the device. Why don't add a test
on flag promiscuous directly in ixgbe_vlan_filter_enable? Or do it on each
call, if we want to allow a device in promiscuous
mode to enable this feature.

What do you think?
    

I can believe that there are paths that lead to this not working
correctly.  That was actually my larger point: this is something that
is commonly not implemented correctly in drivers.  Rather than try to
study every driver my goal is to just avoid the problem completely by
handling vlan acceleration centrally in the networking core.  I sent
out an RFC patch series a few days ago that should solve this problem:

http://marc.info/?l=linux-netdev&m=128700022614170&w=3
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Thank you, I'm going to check these patches and try to apply them in our 
kernel.

Best Regards,

-- 
Guillaume Gaudonville
6WIND
Software Engineer

Tel: +33 1 39 30 92 63
Mob: +33 6 47 85 34 33
Fax: +33 1 39 30 92 11
guillaume.gaudonville@6wind.com
www.6wind.com
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