Re: [PATCH] x86/cpufeatures: Add feature flag for fast short rep movsb

From: Borislav Petkov
Date: Thu Dec 12 2019 - 17:52:20 EST

On Thu, Dec 12, 2019 at 01:49:08PM -0800, Tony Luck wrote:
> From the Intel Optimization Reference Manual:
> Fast Short REP MOVSB
> Beginning with processors based on Ice Lake Client microarchitecture,
> REP MOVSB performance of short operations is enhanced. 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. There is no change
> in the REP STOS performance.
> Add an X86_FEATURE_FSRM flag for this.
> Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>
> ---
> Net effect of this patch is just to make "fsrm" appear in the
> flags section of /proc/cpuinfo. Maybe someone can look into whether
> we should make copy routines that use "rep movsb" check for this
> flag to optimize copies on older CPUs that don't have it?

We can then add the feature flag too. Just showing it in /proc/cpuinfo
without any users is kinda pointless...