If you need Ethernet, good luck. I have been unable to get the Realtek Ethernet devices to function reliably -- barely at all -- on my Lemur, but Wifi works great.
For the error that you're getting, you need to add 03:00.0 to sys-net. However, even with that change, I was only sometimes able to get Ethernet connectivity.
Similar:
https://groups.google.com/forum/#!topic/qubes-users/Fs94QAc3vQI
https://github.com/QubesOS/qubes-issues/issues/1393
I played around with permissive settings but ultimately it is unneeded for Wifi and did not seem to impact the reliability of the Ethernet.
My experience:
Sometimes Ethernet would work only to not work upon reboot, regardless of the kernel version or configurations I attempted; although, my Linux troubleshooting skills are minimal so maybe I missed something in the kernel/xen and other logs. Yet, it seemed strange that it would work during one session but not after a VM and/or system reboot. Something about the 03:00.0/1 device does not play well with Xen/Qubes. So, unless you really need Ethernet, I recommend only connecting your Wifi device to sys-net. The 8260 works great when on its own.
that worked. first machine ive had to manually assign the net device too. mine also didnt work when both devices were assigned to it. ill revisit this at the next stable release.
I finally took some time to troubleshoot the Ethernet issue. To get the Ethernet adapter to work, you need to blacklist the PCI Express Card Reader (RTL8411B) [03:00:0] module (rtsx_pci).
Add "modprobe.blacklist=rtsx_pci" to your <sys-net> kernelopts.
In dom0, you can determine your current kernelopts using qvm-prefs; replace <sys-net> with the name of your sys-net qvm.
[user@dom0 ~]$ qvm-prefs <sys-net> kernelopts
Once you have your current kernelopts, you can just append the modprobe command to it. For example, the following uses values from my system; replace <sys-net> with your sys-net qvm.
[user@dom0 ~]$ qvm-prefs -s <sys-net> kernelopts "nopat iommu=soft swiotlb=8192 modprobe.blacklist=rtsx_pci"
You should now be able to use your Ethernet port in that qvm. Just make sure that both [03:00:0] and [03:00:1] are attached. It will also work with your wireless adapter.
One additional note is that -- at least, on my system -- the qvm takes a bit longer than 20 seconds to shutdown, so you may see one warning about it taking too long. Just select "Wait another 20 seconds" and it should be good.
Ref: https://groups.google.com/forum/#!msg/qubes-devel/logslMTHyW4/khnEzBjAAwAJ
=================================Extra Info
You should be seeing an error in your Xen console qvm log when your kernel attempts to configure the 'rtsx_pci' module.
[ 2.357934] rtsx_pci 0000:00:00.0: Xen PCI mapped GSI16 to IRQ24
[ 2.358347] rtsx_pci 0000:00:00.0: rtsx_pci_acquire_irq: pcr->msi_en = 1, pci->irq = 25
[ 2.358473] BUG: unable to handle kernel paging request at ffffc900002f6010