difference betweeen passthrough NIC and bridge setup in qemu

Question

I set up a quemu virtual machine using virsh in a way that I pass through my physical USB NIC connected to the host machine directly to the virtual machine.

This means that I see my USB NIC in the guest machine with lsusb.

Does this setup(or the one where PCIe pass through for NIC is used) have advantages over more conventional bridged setup where physical NIC of the host machine and virtual machine via tap interface are connected to Linux-bridge?

Does such pass-through setup provide any performance benefits? Are there any practical reasons for such a setup?

Answer 1

A pass-through NIC setup means as if the hardware would be itself connected to the machine instead of being connected to the host machine. Likewise, it will also mean you have to setup the hw and drivers directly in the VM and will be only able to use it from that VM.

This configuration is often used for performance reasons when dealing with GPUs, or for dealing with special uses of wifi chipsets, mainly for hacking purposes.

As for the bridge or NAT setups, multiple VMs are then able to share the network connection/hw of the host, and are giving an abstraction of hardware that does not often map to the brand of the real hardware.

The obvious advantages here are sharing the connection, not having to setup difficult brands in the VM, or even using a more generic abstraction of paravirtualizing hw for the hypervisor//host not have to emulate the full hw, and so not use so much resources.