documentation/hardware.md

How to verify PCIe slot

If you install card in 4x lane PCIe slot, while card designed for 8x PCIe, most likely your card wont be able to reach max bandwidth and will drop packets due PCIe bus bandwidth limit.

pppoe01 ~ # cat /sys/class/net/eth3/device/current_link_width 
8
pppoe01 ~ # cat /sys/class/net/eth3/device/max_link_width 
8

This means card working on maximum PCI lanes

pppoe04 ~ # cat /sys/class/net/eth5/device/current_link_width 
4
pppoe04 ~ # cat /sys/class/net/eth5/device/max_link_width 
8

And here you can see card working in PCIe 4x lanes slot, while it supports 8x.