Re: [PATCH v4 1/5] net: Add support for hardware-offloaded encapsulation

[saeed bishara]

On Mon, Dec 10, 2012 at 9:58 PM, Dmitry Kravkov <dmitry@xxxxxxxxxxxx> wrote:
>> -----Original Message-----
>> From: saeed bishara [mailto:saeed.bishara@xxxxxxxxx]
>> Sent: Monday, December 10, 2012 12:04 PM
>> To: Joseph Gasparakis
>> Cc: davem@xxxxxxxxxxxxx; shemminger@xxxxxxxxxx; chrisw@xxxxxxxxxxxx;
>> gospo@xxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
>> Dmitry Kravkov; bhutchings@xxxxxxxxxxxxxx; Peter P Waskiewicz Jr; Alexander
>> Duyck
>> Subject: Re: [PATCH v4 1/5] net: Add support for hardware-offloaded
>> encapsulation
>>
>> > +static inline struct iphdr *inner_ip_hdr(const struct sk_buff *skb)
>> > +{
>> > +       return (struct iphdr *)skb_inner_network_header(skb);
>> > +}
>>
>> Hi,
>> I'm a little bit bothered because of those inner_ functions, what
>> about the following approach:
>> 1. the skb will have a new state, that state can be outer (normal
>> mode) and inner.
>> 2. when you change the state to inner, all the helper functions such
>> as ip_hdr will return the innter header.
>>
>> that's ofcourse the API side. the implementation may still use the
>> fields you added to the skb.
>>
>> what you think?
>> saeed
>
> Some drivers will probably need both inner_ and other_ in same flow, switching between two states will consume cpu cycles.
from performance perspective, I'm not sure the switching is worse, it
may be better as it reduces code size. please have a look at patch
2/5, with switching you can avoid doing the following change -> less
code, less if-else.
-                               skb_set_transport_header(skb,
-                                       skb_checksum_start_offset(skb));
+                               if (skb->encapsulation)
+                                       skb_set_inner_transport_header(skb,
+                                               skb_checksum_start_offset(skb));
+                               else
+                                       skb_set_transport_header(skb,
+                                               skb_checksum_start_offset(skb));
                                if (!(features & NETIF_F_ALL_CSUM) &&

I think also that from (stack) maintenance perspective, less code is better.
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