[ofa-general] Re: [PATCH - 11] ipoib - add LSO support

[Or Gerlitz]

Michael S. Tsirkin wrote:
>> Quoting Or Gerlitz <ogerlitz at voltaire.com>:

>> With the ipoib connected mode a typical MTU exposed to the OS is 64K, 
>> where data goes over IB RC (soon to be UC) transport meaning that the 
>> HCA does the "fragmentation / reassembly" at the QP (IB L4) level 
>> allowing to send one 64K IB packet over a path whose Link layer (IB L2) 
>> MTU is 2k.

>> So TCP can use segments size upto 64K and UDP can send datagram's whose 
>> size is upto 64K without IP fragmentation.

>> With all this at hand, what does LSO buys you at all?

> If you read these patches closely, you'll see that LSO
> is disabled for connected mode.

This makes me more confused: if someone buys connectX and uses IPoIB, 
since connected mode provides the best performance for both TCP and UDP 
unicast traffic they would not bother to use the datagram mode.

And if they have UDP multicast traffic, they would use the UD QP of 
IPoIB (which means they use datagram mode for multicast).

So the only case where I see usage of the LSO stateless offloading is 
when working against device which does not support connected mode, such 
as IB/Ethernet IP gateways, but as you may know, very much of the 
traffic over these gateways is IP multicast based...

Or.