It’s not too often that “fixes” to the Kernel-based Virtual Machine (KVM) are noteworthy but today is an interesting exception with among the KVM fixes sent in today ahead of the Linux 6.13-rc3 tagging is for beginning to deal with a “hilarious/revolting” performance regression affecting recent generations of Intel processors. This performance regression won’t be fully worked around until Linux 6.14 but at least there is an interim step in place once the code is merged later today.
Catching my attention with this morning’s KVM changes for Linux 6.13-rc3 is this lone KVM x86-related change:
“Cache CPUID.0xD XSTATE offsets+sizes during module init – On Intel’s Emerald Rapids CPUID costs hundreds of cycles and there are a lot of leaves under 0xD. Getting rid of the CPUIDs during nested VM-Enter and VM-Exit is planned for the next release, for now just cache them: even on Skylake that is 40% faster.”
Okay, this is intriguing… Resorting to XSTATE caching to deal with newer Intel Xeon Emerald Rapids CPUs being much more costly than prior processors.
