I recently my physical pfSesne install to a virtual machine. I have tried both pfSense 2.5.6 and 2.6 in kvm and am passing two physical interfaces through, using a Proxmox (debian) host. Each interface is a BCM5709, in a Dell PowerEdge R710.
I have found that in order to work around the issue, I have to disconnect the ethernet cables and allow pfSense to fully boot to the login prompt before reconnecting them, or else all interfaces spam discard frame errors with len 0 and pkt klen 0. Occasionally, even this workaround fails.
This is problematic in the case of a power event, as it means I will need to be physically present for the network to come back up. While I realize I'm thankfully not using this in an enterprise environment, it would be nice to fix this issue if possible.
Original bug on pfSense tracker: https://redmine.pfsense.org/issues/5649
I should mention that I think pfSense is based on 10.1, and not 11 as I mistakenly selected.
As of tonight's nightly build this issue seems to have gone away (pfSense snapshot).
I should note that it was a false positive on the fix being related to anything within FreeBSD or pfSense, for the most part:
After using pci-stub on the Linux host for the two NIC's in question, which prevents the host from loading firmware for the cards (previously wasn't possible for a variety of reasons), coupled with the wiki entry below, I seem to have this issue no longer.
That saves me buying a dual Intel NIC. I hope this information can help others with similar issues.