[SOLVED] PCI passthrough causes Proxmox host to crash

[Zx4ACQQ7]

Hello,
I was trying to install an OPNsense VM on a host with Proxmox.
I installed it fine and then went to configure it and everything was still working.

I noticed that after I added a PCI device (NIC) as passthrough the host started crashing as soon as the VM was booting which means that if you enable it to start up on boot this would restart the host every ~2 minutes.

 

[Zx4ACQQ7]

After a long investigation and trying different things from the internet I finally found the solution.
This happened because I had assigned all of the memory to the VM.
This works fine normally but for some reason the passthrough does not like that and requires some memory for itself.
Once I reduced the memory (16GB -> 12GB with ballooning) and there was some memory left for the host the problem was solved and everything was working.

A few things that I tried which were described as solutions that didn't have an effect in the end are:
1. Enabling/disabling ROM-BAR
2. Leaving one NIC for the host
3. Trying different NIC's just in case the host had earmarked one for itself and it was somehow clashing