feat: jumbo frames on the guest link

docs/GUEST-SETUP.md

@@ -132,6 +132,31 @@ has an in-box driver for the Intel 82576 it emulates. Do not use virtiofs for ho
 virtio device the scanner names, and its shared memory backing blocks transparent hugepages for the
 whole guest. Share over SMB on the `igb` link instead.
 
+### Jumbo frames
+
+`vm-native-setup` puts `<mtu size='9000'/>` on the guest interface. The emulated NIC is packet-rate
+bound rather than bandwidth bound, so larger frames cut the per-packet cost the guest pays on
+receive - the direction that matters when you read files off an SMB share. Measured on igb, host to
+guest: 2922 Mbit/s at 1500, 14232 Mbit/s at 9000, byte-for-byte identical either way.
+
+Both ends have to agree or the frames are simply dropped. The libvirt network needs it too:
+
+```
+virsh net-edit default        # add <mtu size='9000'/>
+virsh net-destroy default && virsh net-start default
+```
+
+and in the guest, set the adapter's *Jumbo Packet* to 9014 and the interface MTU to 9000:
+
+```powershell
+$n = (Get-NetAdapter -Physical | Where-Object Status -eq 'Up').Name
+Set-NetAdapterAdvancedProperty -Name $n -RegistryKeyword "*JumboPacket" -RegistryValue 9014
+Set-NetIPInterface -InterfaceAlias $n -NlMtu 9000
+```
+
+Check `Get-NetIPInterface` afterwards: setting *Jumbo Packet* alone leaves the IP MTU at 1500 and
+you get none of the benefit.
+
 ## 5. Remove the virtio drivers and the agents
 
 Boot the guest. It is on NVMe now, so `viostor` and `vioscsi` can go. In this order, as