[question] extract the feature bits width to 4

[gengdongjiu]

Hi,suzuki/mark,

  very sorry to disturb you, I have a question that want to consult with you. For the CPU feature detection,
why we use extract 4 bits width for the feature match instead of the actual bits number[1]?  may be the actual hardware feature bit more than 4 bits.
thanks!

static inline int __attribute_const__
cpuid_feature_extract_field(u64 features, int field, bool sign)
{
        return cpuid_feature_extract_field_width(features, field, 4, sign);
}

static bool
feature_matches(u64 reg, const struct arm64_cpu_capabilities *entry)
{
        int val = cpuid_feature_extract_field(reg, entry->field_pos, entry->sign);

        return val >= entry->min_field_value;
}