virtio disk host cpu overhead?

[Thomas Fjellstrom <thomas@fjellstrom.ca>]

I've been testing a new samsung 960 evo nvme drive i just got, and running 
tests on it in a guest shows a minimum of double the cpu time on the host as 
it does on the guest. Somehow a guest running a single thread iozone or fio 
test can use 4+ cores on the host side at 50-100%.

I have debian's 4.9.13 kernel installed on the host (4.9.2 on the guest), and 
kvm/qemu 2.8.0. I'm using libvirt and virt-manager to set up the guest, with a 
lvm volume set up on the nvme drive, and exporting a couple volumes to the 
guest which it then either uses lvm on, or testing is done raw on a secondary 
volume. (I've tried both)

Host is a consumer box with an AMD 970 board + FX-8320, and 32GB ram. Guest is 
currently allocated 8 threads, and 1GB ram which it is currently only using 
111MB of while fio is running.

I'm curious to know if this is expected behavior, misconfiguration on my end, 
or a misunderstanding of how things work?

I'd appreciate any insight or help anyone is able and willing to provide.

-- 
Thomas Fjellstrom
thomas@fjellstrom.ca