What is the effect of doubling the number of available PCIe ports (-device pcie-root-port)?

[ILIV]

Hi! 

We've been trying to understand what the effect of increasing number of 
'-device pcie-root-port...' might be in terms of system resources consumption 
and overall system performance. 

In this specific scenario, we're running an oVirt deployment with about 100 
hosts and about 1.3K VMs.

oVirt allocates 16 PCIe ports by default (or is it QEMU defaults?) per VM and 
we would like to increase that number to about 30. 

However, only very few VMs (under 5 out of those ~1.3K) are actually gonna use 
those extra PCIe ports in terms of having additional devices (e.g. NICs). 

So, basically, there will be a lot of VMs that will have about twice as many 
PCIe ports available to them than now but not really using them (as in having 
actual additional devices created and associated with the VMs). 

How will that change load/utilization of resources on the physical servers that 
are running the VMs? Should we expect more memory consumption on the hosts that 
run VMs? If so, how significant of an impact it might be? Will there be a 
noticeably higher CPU utilization?

Are there any potential problems for VMs themselves?

Ivan