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
