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

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Date:   Thu, 12 Dec 2019 23:52:10 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Tony Luck <tony.luck@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/cpufeatures: Add feature flag for fast short rep
 movsb

On Thu, Dec 12, 2019 at 01:49:08PM -0800, Tony Luck wrote:
> From the Intel Optimization Reference Manual:
> 
> 3.7.6.1 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@...el.com>
> ---
> 
> 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...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette