netdev - Re: Trying to implement secondary loopback

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Message-ID: <51433934.3080405@hhi.fraunhofer.de>
Date:	Fri, 15 Mar 2013 16:07:32 +0100
From:	Thomas Martitz <thomas.martitz@....fraunhofer.de>
To:	richard -rw- weinberger <richard.weinberger@...il.com>
CC:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"edumazet@...gle.com" <edumazet@...gle.com>,
	"herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>
Subject: Re: Trying to implement secondary loopback

Am 15.03.2013 15:50, schrieb Thomas Martitz:
> Am 15.03.2013 15:45, schrieb richard -rw- weinberger:
>> On Fri, Mar 15, 2013 at 3:35 PM, Thomas Martitz
>> <thomas.martitz@....fraunhofer.de> wrote:
>>> Am 15.03.2013 15:32, schrieb richard -rw- weinberger:
>>>
>>>> On Fri, Mar 15, 2013 at 3:20 PM, Thomas Martitz
>>>> <thomas.martitz@....fraunhofer.de> wrote:
>>>>>>
>>>>>> The real question is, why do you need a second one?
>>>>>> I assume your driver (for the non-existing hardware) is a ethernet
>>>>>> driver,
>>>>>> and now you're looking for a way to test your shiny new ethX device,
>>>>>> correct?
>>>>>>
>>>>>
>>>>> Yes. But that's only the first step. Once I have the basic ethX device
>>>>> working I want to make sure the PCIe data transfer is working (with
>>>>> good
>>>>> performance), ideally using the very same test application in user
>>>>> space.
>>>>
>>>>
>>>> And there is the second loopback device is this picture?
>>>>
>>>
>>> Mine is the second one, as I cannot modify "lo". The standard "lo"
>>> interface
>>> is the the first loopback and it seems to be hardcoded to be the only
>>> one.
>>
>> And why can't you implement a regular ethernet driver like everyone
>> else does?
>>
>
>
> That was actually my first attepmt, because I was sure it would work.
> However when I tried that I had trouble to get transfers on localhost
> routed to my code. I did "ifconfig lo down" but then it didn't transfer
> at all. I will retry.
>


Same result. I assumed the kernel treats lo in a special way for 
localhost-connections and that it would be impossible to achieve the 
same with a custom interface.

I did the following:

ifconfig lo down
insmod ./mykmod.ko
ifconfig eth2 up
ifconfig eth2 127.0.0.1

At this point ifconfig prints the same information for eth2 that it had 
printed for lo before (except for the LOOPBACK flag, but I can enable 
that one as well by adding IFF_LOOPBACK to the interface flags in the 
module). Yet my test application only works with lo, not eth2.

Best regards.

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