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[Xen-devel] Xen & AoE problem = Ethernet padding

  • To: xen-devel@xxxxxxxxxxxxxxxxxxx
  • From: Marcelo Messa <mrmessa@xxxxxxxxx>
  • Date: Fri, 25 Jan 2008 00:27:09 -0200
  • Cc: Marcelo Messa <mrmessa@xxxxxxxxx>
  • Delivery-date: Mon, 28 Jan 2008 02:32:33 -0800
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Hi there,

         This is not a "broadcast", but also, a kind of ping.

I have an entry at http://bugzilla.xensource.com/bugzilla/ show_bug.cgi?id=1140 , and maybe some of you could be interested.

          It follows:

I made some tests about ethernet padding ( an expected behavior from ethernet 10/100 on-wire, that needs at least 60 bytes of data to be able to CRC. NIC chipset has the job of fullfill the packet if it is smaller than this - also called "padding" ) and I notice a behavior that is not expected (at
least for me).

>>> Ethernet Padding Sender  /  Receiver Matrix <<<

      \RECEIVER  | Dom0_vif |FullV_DomU_2|ParaV_DomU_2|SMP_ParaV_DomU_2
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)

My setup is a Centos 5.1 (2.6.18-53.1.4.el5xen) / Xen (3.0.3 or 3.1, as I hear) in a
bridge network configuration.

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.

A direct related problem: it's not possible to export AoE disks from Dom0 and other Paravirtualized guests (AoE blade server verifies if the packet has a minimal size - 60 bytes) to any local VM. 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.

Best Regards
      Marcelo Messa

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