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

[Andrew Cooper <andrew.cooper3@citrix.com>]

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.

>
> Check 2 (Based on MISC_ENABLE.FAST_STRING):
>> /*
>>  * If fast string is not enabled in IA32_MISC_ENABLE for any reason,
>>  * clear the fast string and enhanced fast string CPU capabilities.

I'd suggest that a better way of phrasing this is:

/* BIOSes typically have a knob for Fast Strings.  Honour the user's
wishes. */

>>  */
>> if (c->x86_vfm >= INTEL_PENTIUM_M_DOTHAN) {
>> 	rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
>> 	if (misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING) {
>> 		/* X86_FEATURE_ERMS will be automatically set based on CPUID */
>> 		set_cpu_cap(c, X86_FEATURE_REP_GOOD);
>> 	} else {
>> 		pr_info("Disabled fast string operations\n");
>> 		setup_clear_cpu_cap(X86_FEATURE_REP_GOOD);
>> 		setup_clear_cpu_cap(X86_FEATURE_ERMS);
>> 	}
>> }

MSR_MISC_ENABLE exists on all 64bit CPUs, and some 32bit ones too. 
Therefore, this section alone seems to suffice in order to set up
REP_GOOD properly.

~Andrew