The computing community is full of concern for the Meltdown vulnerability. Page-Table Isolation is the name of the patch. Many workloads can see a performance hit. How is GPU Passthrough affected? These benchmarks will attempt to show this.

An Overview

Meltdown is an architectural vulnerability in Intel and certain ARM CPUs. It allows an attacker to read the memory of a process it should not have access to. As a result of its discovery, Microsoft and the Linux kernel team have issued a patch called page-table isolation. The patch’s performance impact affects applications which use a large number of syscalls. The issue with virtualization is that you have two kernels. Both the host and guest kernel need patches. It is interesting that in most workloads the impact was almost nothing with KVM. The PCID x86 extension helps to mitigate the performance impact significantly. It is found on Intel Haswell and above CPUs.

The Benchmarked System

The following hardware hosted these benchmarks:

Intel Core i5-6500 CPU (vulnerable to Meltdown)

Asus H110M-A/M.2

2x8GB EVGA DDR4 @ 2133MHz

500GB Samsung 850 Evo SSD

2TB Seagate BarraCuda 7200RPM disk

Corsair CX600M power supply

Gigabyte G1 Gaming GTX 1060 3GB

The Benchmarked Software

The following software hosted these benchmarks:

Debian Unstable/Sid

Linux 4.14.12

QEMU 2.11.0

Libvirt 3.10.0

Windows 10 1703 (Creators Update) guest

CPU Performance

CPU benchmarks such as Cinebench and Blender saw little to no performance impact. Blender used the AMD Ryzen Blender file. This result is unsurprising as these applications do not require very many syscalls. As a result, people who do productivity in their virtual machine should not see any difference.

Disk I/O

Disk I/O is one area where a massive performance hit would be expected. However, the VirtIO Windows drivers are doing very well here. The page-table isolation patch sees almost no difference in performance across the board. Therefore your boot times and game load times should not see any impact. I ran the benchmarks on the 850 Evo drive.

Some Games

I picked these games as they are a good mix of stressing the CPU and GPU. These results confirm that games should see an almost zero impact by these patches. Noteworthy is the Rainbow Six Siege minimum result. It was almost 7 frames per second lower with page-table isolation enabled.

Conclusion

Many people have blown the performance impacts out of proportion. For almost all GPU Passthrough users, no performance hit should be observed. VirtIO drivers possibly help with this. It also seems like Microsoft may have added some additional optimizations with the page-table patch to counteract the performance loss.

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