CPU Copy Host Configuration (CPU Passthrough) does not work.

[Robert Dinse]

I have a Windows 10 guest with using CPU passthrough and UHD850 i915 virtualized GPU passthrough. I had not been getting the performance I had expected but the issue was with CPU not GPU.

The host machine is an Asus motherboard with an i7-9700k CPU. This is an 8-core CPU with 1 thread per core.

I installed CPU-Z on the Windows guest, this is a utility from Asus that gives info about the CPU in a fair amount of detail. Instead of showing one CPU socket, 8 cores, and 1 thread per core, it was showing 2 sockets, 1 core per socket, and 1 thread per core, hence only 1/4th of the available CPU cycles were available to Windows and even this not correctly.

By clicking on Manually Set CPU topology and selecting the correct configuration, the CPU gets passed through properly and Windows sees all eight cores on a single socket as it should.

As I also run an ISP hosting service on Intel platforms and primarily utilize Ubuntu 22.04 for that as well, I thought it would be interesting to see how this affected web guests. My web server was perhaps an acid test for this as it is fairly heavily traffic'd with a lot of PHP and database centric applications.

The web server runs on an i7-6850k physical host, this is a six core 12 thread CPU, and while Linux could see all the separate threads, it saw them as 12 1 core CPUs. Curious to see if there was any performance penalty for this I timed the page loading time of our wordpress site, both with the default configuration which gives linux 12 1 core CPUs and the correct configuration of 1 socket, six cores, and 2 threads per core. The former yielded an average page load time of 240ms, the latter 80ms, a THREE FOLD improvement.

So while this is a bug I can manually work around and thus not a high priority one, it does substantially impact the performance of Linux with kvm/qemu guests and thus it would be advantageous to Ubuntu to fix this.

ProblemType: Bug
DistroRelease: Ubuntu 22.04
Package: virt-manager 1:4.0.0-1
Uname: Linux 5.17.15 x86_64
ApportVersion: 2.20.11-0ubuntu82.1
Architecture: amd64
CasperMD5CheckResult: unknown
CurrentDesktop: MATE
Date: Sat Jul 9 23:08:04 2022
PackageArchitecture: all
SourcePackage: virt-manager
UpgradeStatus: No upgrade log present (probably fresh install)