[ovs-discuss] discuss Digest, Vol 30, Issue 5

[likunyun]

In the kernel, Xen's checksum fields are defined in sk_buff, and CONFIG_XEN is defined in .config.
They are  used in many place , like in the front of dev_queue_xmit():
... ... 
if (!skb->proto_csum_blank)
  return 0;
 
skb_checksum_setup()
... 
 
Kernel is is using the xen model.
 
> From: jesse at nicira.com
> Date: Thu, 15 Dec 2011 18:57:05 -0800
> Subject: Re: [ovs-discuss] discuss Digest, Vol 30, Issue 5
> To: kunyunli at hotmail.com
> CC: discuss at openvswitch.org
> 
> 2011/12/15 likunyun <kunyunli at hotmail.com>:
> >> The URL that you provided requires a username and password to login.
> > The account is owned by my company, Maybe I could send the code after I was
> > allowed taking the code home, it will take serveral days.
> > Or you figure out the information you care, then I provide them.
> >
> > OVS finish the skb_checksum_setup as soon as it gets packetes in kernel.
> > netdev_port_receive() ->compute_ip_summed() ->vswitch_skb_checksum_setup()
> > The skb->csum is setted to be csum_offset in vswitch_skb_checksum_setup().
> > After the flow being created, the skb->csum wouldn't be corrected in
> > kernel except ovs finish the tso .
> >
> > Why it set the wrong date in sk_buff in the compatibility code for older
> > kernels ?
> 
> Older versions of Linux did not distinguish between checksums that
> have been verified in hardware on receive and those that need to be
> computed in hardware on transmit. The meanings of these two are very
> different but it's generally not ambiguous because you know whether
> you are on the transmit or receive path. The exception to this is
> bridging and so Xen added some new fields to disambiguate between
> them. Linux later added fields for the same purpose but different
> from the ones that Xen added. OVS attempts to use the correct ones
> depending on the situation.
> 
> Your kernel is apparently defining both but is using the Linux model.
> Other Xen patched modern kernels with both generally use the Xen
> version, so that's what OVS is trying to do. The question that needs
> to be answered is whether the proto_data_valid and proto_csum_blank
> fields in struct sk_buff are actually used anywhere or are just
> defined (those are Xen's checksum fields). If the answer is that
> they're not used then we need to come up with a test that excludes
> your kernel but catches the others.
 		 	   		  
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