Amazon’s physical hardware and EC2 compute unit « Huan Liu's Blog

Amazon’s physical hardware and EC2 compute unit « Huan Liu's Blog

CPUID
The “cpuid” instruction is supported by all x86 CPU manufacturers, and it is designed to report the capabilities of the CPU. This instruction is non-trapping, meaning that you can execute it in user mode without triggering protection trap. In the Xen paravirtualized hypervisor (what Amazon uses), it means that the hypervisor would not be able to intercept the instruction, and change the result that it returns. Therefore, the output from “cpuid” is the real output from the physical CPU.
We look at several fields in the “cpuid” output. First and foremost, we look at the branding string, which identifies the CPU’s model number. We also look at “local APIC physical ID” in (1/ebx). The “APIC ID” is unique for a physical core. By enumerating all “APIC ID”s, we know how many physical cores are there. Lastly, we look at “Logical CPU cores” in (0×80000008/ecx). It is supposed to show how many hyper-thread cores are on a physical core.