Performance Impact of Host Kernel Page Table Isolation on Virtualized Servers

As Meltdown mitigation, Kernel Page Table I solation (KPTI) was merged into Linux kernel mainline, and the performance impact is significant on x86 processors. Most of the previous work focuses on how KPTI affects Linux kernel performance within the scope of virtual machines or physical machines on x86. However, whether host KPTI affects virtual machines has not been well studied. What's more, there is relatively little research on ARM CPUs. This paper presents an in-depth study of how KPTI on the host affects the virtualized server performance and compares ARMv8 and x86. We first run several application benchmarks to demonstrate the performance impact does exist. The reason is that with a para-virtual I/O scheme, guest offloads I/O requests to the host side, which may incur user/kernel transitions. For the network I/O, when using QEMU as the back-end device, we saw a 1.7% and 5.5% slowdown on ARMv8 and x86, respectively. vhost and vhost-user, originally proposed to optimize performance, inadvertently mitigate the performance impact introduced by host KPTI. For CPU and memory-intensive benchmarks, the performance impact is trivial. We also find that virtual machines on ARMv8 are less affected by KPTI. To diagnose the root cause, we port HyperBench to the ARM virtualization platform. The final results show that swapping the translation table pointer register on ARMv8 is about 3.5x faster than x86. Our findings have significant implications for tuning the x86 virtualization platform's performance and helping ARMv8 administrators enable KPTI with confidence.