Re: [PATCH v8 2/3] x86/cpu: Check if feature string is non-zero

[Sohil Mehta]

On 3/2/2026 7:25 AM, Maciej Wieczor-Retman wrote:

> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 9aa11224a038..b60269174d95 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -675,6 +675,7 @@ cpuid_dependent_features[] = {
>  static void filter_cpuid_features(struct cpuinfo_x86 *c, bool warn)
>  {
>  	const struct cpuid_dependent_feature *df;
> +	char feature_buf[16];
>  

The usage of number 16 isn't intuitive here. Might be useful to use a
macro here. See below..

>  	for (df = cpuid_dependent_features; df->feature; df++) {
>  
> @@ -697,7 +698,7 @@ static void filter_cpuid_features(struct cpuinfo_x86 *c, bool warn)
>  			continue;
>  
>  		pr_warn("CPU: CPU feature %s disabled, no CPUID level 0x%x\n",
> -			x86_cap_flags[df->feature], df->level);
> +			x86_cap_name(df->feature, feature_buf), df->level);
>  	}
>  }
>  
> @@ -1634,6 +1635,7 @@ static inline bool parse_set_clear_cpuid(char *arg, bool set)
>  
>  	while (arg) {
>  		bool found __maybe_unused = false;
> +		char name_buf[16];
>  		unsigned int bit;
>  
>  		opt = strsep(&arg, ",");
> @@ -1654,10 +1656,7 @@ static inline bool parse_set_clear_cpuid(char *arg, bool set)
>  					setup_clear_cpu_cap(bit);
>  				}
>  				/* empty-string, i.e., ""-defined feature flags */
> -				if (!x86_cap_flags[bit])
> -					pr_cont(" %d:%d\n", bit >> 5, bit & 31);
> -				else
> -					pr_cont(" %s\n", x86_cap_flags[bit]);
> +				pr_cont(" %s\n", x86_cap_name(bit, name_buf));
>  
>  				taint++;
>  			}
> @@ -1980,6 +1979,23 @@ static void generic_identify(struct cpuinfo_x86 *c)
>  #endif
>  }
>  

It would be useful to have a comment here because the function is called
from multiple files. You can probably reuse the one from the deleted
x86_feature_name().

/*
 * Return the feature "name" if available, otherwise return
 * the X86_FEATURE_* numerals to make it easier to identify
 * the feature.
 */

> +const char *x86_cap_name(unsigned int bit, char *buf)
> +{
> +	unsigned int word = bit >> 5;
> +	const char *name = NULL;
> +
> +	if (likely(word < NCAPINTS))
> +		name = x86_cap_flags[bit];
> +	else if (likely(word < NCAPINTS + NBUGINTS))
> +		name = x86_bug_flags[bit - 32 * NCAPINTS];
> +
> +	if (name)
> +		return name;
> +
> +	snprintf(buf, 16, "%u:%u", word, bit & 31);

The buffer size of 16 is expected to be in sync with the callers and
easy to mess up. Also, the callers wouldn't know that it needs to be 16
because of this snprintf(). Should we have a simple define for it?

Maybe something like,

#define X86_CAP_BUF_SIZE 16

> +	return buf;
> +}
> +
>  /*
>   * This does the hard work of actually picking apart the CPU stuff...
>   */
> diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
> index 146f6f8b0650..dfd79b06ab7b 100644
> --- a/arch/x86/kernel/cpu/cpuid-deps.c
> +++ b/arch/x86/kernel/cpu/cpuid-deps.c
> @@ -2,6 +2,7 @@
>  #include <linux/kernel.h>
>  #include <linux/init.h>
>  #include <linux/module.h>

Nit:

You can consider adding a newline here.

> +#include <asm/cpu.h>
>  #include <asm/cpufeature.h>
>