Problem with netconsole and eth0 timing

[valdis.kletnieks@vt.edu (valdis.kletnieks at vt.edu)]

On Wed, 26 Sep 2018 13:25:35 -0700, don fisher said:
> Would you tell me how to tell the driver that it is to be eth0, ip
> address etc. Maybe on linux command line, but I do not know the format.

To quote some guy named Don Fisher:

> my kernel and including the proper command (as shown below) in the linux boot string:
>   netconsole=64001 at 192.168.7.60/eth0,64001 at 192.168.7.55/34:e6:d7:01:2a:dd

That's how. The netconsole command gets the info it needs from there, and tells
the network layer how to configure the ethernet device and the network layer -
although mostly the network layer.  And the devices will auto-name themselves
during boot, so all you need to do is know *which* name the kernel gives to the
port you want to use, and then use that name.

So grovel around in dmesg, and look for lines like (2 examples I have handy here)

 grep eth /var/log/dmesg
[    7.278395] igb 0000:07:00.0: added PHC on eth0
[    7.278398] igb 0000:07:00.0: eth0: (PCIe:5.0Gb/s:Width x2) 24:6e:96:10:db:64
[    7.278916] igb 0000:07:00.0: eth0: PBA No: G61346-000
[    7.368911] igb 0000:07:00.1: added PHC on eth1
[    7.368913] igb 0000:07:00.1: eth1: (PCIe:5.0Gb/s:Width x2) 24:6e:96:10:db:65
[    7.369372] igb 0000:07:00.1: eth1: PBA No: G61346-000

dmesg | grep eth
[    2.642006] e1000e 0000:00:19.0 eth0: (PCI Express:2.5GT/s:Width x1) f0:1f:af:0c:8a:da
[    2.642076] e1000e 0000:00:19.0 eth0: Intel(R) PRO/1000 Network Connection
[    2.642118] e1000e 0000:00:19.0 eth0: MAC: 10, PHY: 11, PBA No: 7011FF-0FF
[    5.071095] e1000e 0000:00:19.0 eno1: renamed from eth0
[   44.516004] e1000e 0000:00:19.0 eth0: renamed from eno1

And find the one that has the MAC address of the port you want to use. Note
that you want the kernel-assigned device name, not the one that udev/systemd
finally assign to the device. So for the second example (my laptop), you'd want
either the eth0 or eno1 name (depending which one was in effect when the
netconsole module initializes.  The initial eth0 is from pci enumeration, the
rename to eno1 is courtesy of the kernel, and then the rename back to eth0 by
systemd.  Some experimentation may be needed (I've got a few servers that have
4 1G ports on the motherboard and multiple 10G/40G dual-port cards, so
sometimes the port wired to our management network ends up at eth6 or eth7...)

So if the port you want to use gets named eth4 by the kernel, you use
netconsole=64001 at 192.168.7.60/eth4,64001 at 192.168.7.55/34:e6:d7:01:2a:dd

(Gory ethernet details follow :)

Remember that strictly speaking, the ethernet device itself doesn't *need* to
know what its IP address is - it only needs to know its own MAC address so it
knows which packets on the wire to accept to hand to the network stack, and
*maybe* a list of other MAC addresses it should accept.

And the hardware already knows its own MAC address.. :)

You can get ethernet devices working with a *very* small set of functions:

0) Tell the kernel your hardware state (link/no link, MAC address, a few other things)
1) Receive packets for your own MAC address
2) Receive broadcast packets
3) Receive packets for another specified MAC address (semi-optional)
4) Receive packets in promiscuous mode (semi-optional)
5) Transmit packet to the MAC address provided

Pretty much everything else can be done in kernelspace (though modern
cards often provide offload of some IP and even TCP handling, interrupt
coalescing, and all sorts of other stuff)

(I learned far too much about minimalist Ethernet when the Clarkson Packet
Drivers were getting created in the cubicle next to mine. :)


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