linux-kernel - Re: [PATCH v2 05/17] x86/cpu/intel: Fix page copy performance for extended Families

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Message-ID: <2299c94f-aa46-47b5-bd25-9436a8fbd619@citrix.com>
Date: Wed, 12 Feb 2025 00:54:52 +0000
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Dave Hansen <dave.hansen@...el.com>, Sohil Mehta <sohil.mehta@...el.com>,
 x86@...nel.org, Dave Hansen <dave.hansen@...ux.intel.com>,
 Tony Luck <tony.luck@...el.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>,
 Kan Liang <kan.liang@...ux.intel.com>, Thomas Gleixner <tglx@...utronix.de>,
 Borislav Petkov <bp@...en8.de>, "H . Peter Anvin" <hpa@...or.com>,
 "Rafael J . Wysocki" <rafael@...nel.org>, Len Brown <lenb@...nel.org>,
 Andy Lutomirski <luto@...nel.org>, Viresh Kumar <viresh.kumar@...aro.org>,
 Fenghua Yu <fenghua.yu@...el.com>, Jean Delvare <jdelvare@...e.com>,
 Guenter Roeck <linux@...ck-us.net>, Zhang Rui <rui.zhang@...el.com>,
 David Laight <david.laight.linux@...il.com>,
 linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-acpi@...r.kernel.org, linux-pm@...r.kernel.org,
 linux-hwmon@...r.kernel.org
Subject: Re: [PATCH v2 05/17] x86/cpu/intel: Fix page copy performance for
 extended Families

On 11/02/2025 8:53 pm, Dave Hansen wrote:
> On 2/11/25 11:43, Sohil Mehta wrote:
>> +	/*
>> +	 * Modern CPUs are generally expected to have a sane fast string
>> +	 * implementation. However, the BIOS may disable it on certain CPUs
>> +	 * via the architectural FAST_STRING bit.
>> +	 */
>> +	if (IS_ENABLED(CONFIG_X86_64) && (c->x86 == 6 || c->x86 > 15))
>> +		set_cpu_cap(c, X86_FEATURE_REP_GOOD);
> I'm not sure the BIOS comment is helpful here.
>
> Also, at this point, let's just make the check >=6 (or the >=PPRO
> equivalent).
>
> It will only matter if *all* of these are true:
> 1. Someone has a 64-bit capable P4 that powers on
> 2. They're running a 64-bit mainline kernel
> 3. String copy is *actually* slower than the alternative
> 4. They are performance sensitive enough to notice
>
> We don't even know the answer to #3 for sure. Let's just say what we're
> doing in a comment:
>
> 	/* Assume that any 64-bit CPU has a good implementation */

If you're going to override the BIOS setting, then you need to
explicitly set MSR_MISC_ENABLE.FAST_STRINGS.

Otherwise you're claiming to Linux that REP is good even when hardware
is prohibited from using optimisations.

~Andrew