Comments on: CPU feature flags and their meanings

By: sven

sven — Wed, 17 Oct 2007 16:18:29 +0000

Thanks Steve. Added it to the table. (HVM, and VMX, SVM was there already). Added cross-references to all three.

By: Steve

Steve — Wed, 17 Oct 2007 15:56:58 +0000

Sorry I got that wrong. The flag for Intel is ‘vmx’.

(HVM is what this kind of stuff is called under Xen so I got confused for a second!)

By: Steve

Steve — Wed, 17 Oct 2007 15:55:17 +0000

Don’t forget ‘HVM’ as Intels alternative to AMDs ‘SVM’ flag.

By: sven

sven — Wed, 17 Oct 2007 15:54:36 +0000

I’ve also seen machines without active Hyperthreading but with the HT flag. My VDR machine is the prime example for how this happens: The BIOS doesn’t support Hyperthreading with the CPU I’m using (Pentium E2160), though the CPU does support it.
Your technique obviously works to detect active hyperthreading support.

By: Ken Bloom

Ken Bloom — Wed, 17 Oct 2007 15:50:03 +0000

I have seen hyperthreaded machines and non-hyperthreaded machines that have the ht flag, and have seen that hyperthreaded machines do not have the htt flag.

The best way to detect a hyperthreaded machine is to look for two single-core processors with the same physical id, like the abbreviated /proc/cpuinfo that I’ve included from a dual-core hyperthreaded machine.

processor : 0
physical id : 0
core id : 0
cpu cores : 1

processor : 1
physical id : 0
core id : 0
cpu cores : 1

processor : 2
physical id : 3
core id : 0
cpu cores : 1

processor : 3
physical id : 3
core id : 0
cpu cores : 1