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

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Message-ID: <066c8d9b-05fa-4380-ab2c-91a5e1475021@intel.com>
Date: Thu, 13 Feb 2025 16:29:29 -0800
From: Sohil Mehta <sohil.mehta@...el.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>, Dave Hansen
	<dave.hansen@...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 2/13/2025 3:02 PM, Andrew Cooper wrote:
> On 12/02/2025 9:19 pm, Sohil Mehta wrote:
>> Check 1 (Based on Family Model numbers):
>>> /*
>>>  * Unconditionally set REP_GOOD on early Family 6 processors
>>>  */
>>> if (IS_ENABLED(CONFIG_X86_64) &&
>>>     (c->x86_vfm >= INTEL_PENTIUM_PRO && c->x86_vfm < INTEL_PENTIUM_M_DOTHAN))
>>> 	set_cpu_cap(c, X86_FEATURE_REP_GOOD);
>> This check is mostly redundant since it is targeted for 64 bit and very
>> few if any of those CPUs support 64 bit processing. I suggest that we
>> get rid of this check completely. The risk here is fairly limited as well.
> 
> PENTIUM_PRO is model 0x1.  M_DOTHAN isn't introduced until patch 10, but
> is model 0xd.
> 
> And model 0xf (Memron) is the first 64bit capable fam6 CPU, so this is
> dead code given the CONFIG_X86_64 which the compiler can't actually
> optimise out.
> 

Thanks for confirming. I figured this is likely dead code but wasn't
completely sure.