[Xen-users] Xen () ethernet padding

[Marcelo Messa <mrmessa@xxxxxxxxx>]

Hi,

       I made some tests about ethernet padding ( in my dummy works:  
an expected behavior from ethernet 10/100 on-wire, that needs at  
least 60 bytes of data to be able to CRC and the NIC chipset has the  
job of fullfill the packet if it is smaller than this- padding ) and  
I notice a behavior that is not expected (at least for me).
       My setup is a Centos 5.1/Xen (3.0.3) in a bridge network  
configuration and I've got the following results:
>>> Ethernet Padding Sender  /  Receiver Matrix <<<

                 \RECEIVER | Dom0 vif |  FullV_DomU_2 | ParaV_DomU_2  
| SMP_ParaV_DomU_2
SENDER/
Dom0 vif.......................|   No (OK)   |  Yes (OK) |  No (NOK)  
| No (NOK)
FullV_DomU................|  No (NOK) |  Yes (OK) |  No (NOK) | No (NOK)
ParaV_DomU..............|  No (NOK) |  Yes (OK) |  No (NOK) | No (NOK)
SMP_ParaV_DomU...|  No (NOK) |  Yes (OK) |  No (NOK) | No (NOK)

          The results shows that only FullVirtualization do padding,  
and only when receives a packet (not when send it).
          You could ask "why padding if you dont have a physical  
transport?". If you are playing with some L2 protocol (like AoE),  
this  behavior do impact.
           From results, we conclude that is not possible, for  
example, to export AoE disks from Dom0 and other Paravirtualized  
guests (AoE blade server verifies if the packet has a minimal size -  
60 bytes minimal ethernet on-wire packet). I do not known if another  
layer2 protocols has the same problem, but I think that is a expected  
feature that a virtual NIC works as a physical one..
          To have a consistent behavior,  everybody should do  
ethernet padding (my expected behavior), or no one should do this.
          Make sense? I changed the AoE initiator vblade to not  
discard packets "not padded" (very simple change, but I still dont  
have an answer why the padding limit was there),  but I appreciate if  
someone could give more 2 cents for this issue.
Cheers
      Marcelo Messa 
 

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