[wtallis]

That feature flag is merely a macro in the Linux kernel source code, and doesn't appear to be exposed to userspace. It is entirely different from the kind of flags under discussion, which are in the return values of the CPUID instruction and available for any program to query.

[gpderetta]

from the patch:

The enhancement applies to string lengths between 1 and 128 bytes long. Support for fast-short REP MOVSB is enumerated by the CPUID feature flag: CPUID [EAX=7H, ECX=0H).EDX.FAST_SHORT_REP_MOVSB[bit 4] = 1

So there is indeed a CPUID feature flag for fast rep movsb.