/proc/cpuinfo CPU flags?

Ever wonder what those CPU flags meant when looking at /proc/cpuinfo?

Check out cpufeature.h under /usr/src/kernels/<kernel>/include/

It’ll give you a basic description of what you’re looking at.

$ pwd
/usr/src/kernels/2.6.18-92.el5-i686/include
$ find . -name cpufeature.h
./asm-i386/cpufeature.h

$ grep flags /proc/cpuinfo
flags           : fpu tsc msr pae mce cx8 apic mtrr mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe cid xtpr

So what is fxsr?

$ grep -i fxsr /usr/src/kernels/2.6.18-92.el5-i686/include/asm-i386/cpufeature.h
#define X86_FEATURE_FXSR        (0*32+24) /* FXSAVE and FXRSTOR instructions (fast save and restore */
/* of FPU context), and CR4.OSFXSR available */