Created attachment 1579987 [details]
Performance results which indicate that virtio "passthrough" is not equivalent to true passthrough

Description of problem:
virt-manager allows the user to add a network device with virtio in "passthrough" mode, but passthrough mode isn't full PCI passthrough, and some latency is introduced as a consequence. If instead the user decides to add a PCI host device, this is "true" PCI passthrough and functions appropriately. Some warning should be made that virtio "passthrough" is not equivalent to true PCI passthrough performance.

Version-Release number of selected component (if applicable):
1.4.0 is the virt-manager I'm running

How reproducible:
Very, if the host supports VTx, support is enabled in the BIOS, and the appropriate kernel flag is specified at boot (intel_iommu=on for Debian hosts)

Steps to Reproduce:
1. Open virt-manager
2. Start VM
3. Open hardware
4. Remove existing network device
5. Add Network device, select NIC from dropdown menu, select virtio from dropdown menu
6. Observe config in XML via virsh:
...
    <interface type='direct'>
      <mac address='52:54:00:f0:d0:cd'/>
      <source dev='enp5s0' mode='passthrough'/>
      <model type='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>
...
7. Ping some external host, observe latency.
8. Remove network device, add PCI Host device, select NIC from menu, click finish
9. Observe config:
...
    <hostdev mode='subsystem' type='pci' managed='yes'>
      <source>
        <address domain='0x0000' bus='0x05' slot='0x00' function='0x0'/>
      </source>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x09' function='0x0'/>
    </hostdev>
...
10. Reboot VM, test latency. 
11. Observe that this config, true PCI passthrough, is significantly faster.

I've attached 300 samples of pings sent to a Gb connected server for native pings from host OS, true PCI passthrough pings, virtio "passthrough" pings, and virtual network pings.

Actual results:
Performance which is not representative of true PCI-passthrough

Expected results:
Either latency which is representative of full PCI-passthrough, or some label other than simply "passthrough". This word is misleading.

Additional info: