VirtIO Driver Support

The FreeBSD kernel used by pfSense includes VirtIO drivers built into the kernel. No special action is necessary to enable the drivers.

Disable Hardware Checksum Offloading

With the current state of VirtIO network drivers in FreeBSD, it is necessary to check Disable hardware checksum offload under System > Advanced on the Networking tab and to manually reboot pfSense after saving the setting, even though there is no prompt instructing to do so to be able to reach systems (at least other VM guests, possibly others) protected by pfSense directly from the VM host.

The issue is most likely caused by FreeBSD Bug 165059.

Also, there is a known issue with BGP MD5 sessions and TCP-MD5 when Hardware Checksum Offloading (rxcsum) is disabled. See Bug #8407 and Bug #7969. With the inclusion of FreeBSD 11.2-RELEASE in pfSense 2.4.4 this issue is corrected. See FreeBSD Bugzilla - Bug 223835.

Hardware checksums and other NIC offloading features like TSO may also need to be disabled on the hypervisor system in addition to the pfSense VM.