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Date: Mon, 5 Jan 2015 18:05:30 +0100
From: Luigi Rizzo <rizzo@iet.unipi.it>
To: Adrian Chadd <adrian@freebsd.org>
Subject: Re: netmap over virtio giving packets with extra 12 bytes
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Cc: Avinash Sridharan <avinash.sridharan@gmail.com>,
 Vincenzo Maffione <v.maffione@gmail.com>,
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On Mon, Jan 05, 2015 at 08:33:17AM -0800, Adrian Chadd wrote:
> ... surely virtio should be skipping over those bytes in the netmap rx
> side before handing them up?
> 
> (It won't be the only hardware that puts the RX descriptor status in
> the RX frame itself..)

it is not the rx descriptor, those 12 bytes are also present on the tx side.
Think of them as an encapsulation whose presence is negotiated when KVM connects
to the TAP port, and after that is present on all packets bidirectionally.

Now, surely we could add/remove those bytes in the virtio-netmap code
(at the price of an additional data copy).

I need to investigate further.
Avinash, could you tell us exactly your configuration -- what is the
network backend for QEMU/KVM, and whether you are using virtio in
native or emulated netmap mode ?

cheers
luigi