QEMU supports two x86 chipsets it can emulate. The very ancient (1996) i440FX chipset and the more recent (2007) Q35 chipset. For a long time, Q35 was advised against for GPU passthrough because some parts weren’t quite fully addressed. On the other hand, the advantage of Q35 is that it supports PCIe natively. i440FX only supports PCI and all passed through hardware had to pretend to be PCI rather than PCIe. In theory, this means that Q35 should lead to a slightly lower emulation overhead, so I decided to test it between different workstation seats on my virtualized workstation server.
Initial results are quite interesting. I left both VMs idle after booting with as close to identical setup as there can be (other than Q35 vs i440FX). After around 3 hours, I measured the total CPU time spent while idling. These are two otherwise identical VMs sitting at their login screens after booting:
So it looks like the VM based on Q35 emulated chipset used about 23% less CPU while idling. I haven’t noticed any performance difference by the seat of my pants after conversion to Q35. I haven’t actually run any other benchmarks yet, but a 23% CPU saving with Q35 when idling seems significant. Especially if you have a lot of Windows VMs running that are sitting idle most of the time.