Re: [PATCH v8 3/3] x86/cpu: Do a sanity check on required feature bits

[Sohil Mehta <sohil.mehta@intel.com>]

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

> +/*
> + * As a sanity check compare the final x86_capability bitmask with the initial
> + * predefined required feature bits. In case of a mismatch emit a warning with
> + * the faulty bitmask value.

Aren't we printing the faulty feature name instead of the bitmask value?

> + */
> +static void verify_required_features(const struct cpuinfo_x86 *c)
> +{
> +	u32 missing[NCAPINTS] = REQUIRED_MASK_INITIALIZER;
> +	char cap_buf[16];
> +	u32 error = 0;
> +	unsigned int i;
> +

X86 prefers reverse Xmas order for variable declarations.

> +	for (i = 0; i < NCAPINTS; i++) {
> +		missing[i] &= ~c->x86_capability[i];
> +		error |= missing[i];
> +	}
> +
> +	if (!error)
> +		return;		/* All good */
> +

The tail comments should be avoided. This one is completely unnecessary
here.


> +	/*
> +	 * At least one required feature is missing. Print a warning,
> +	 * and taint the kernel.
> +	 */

The "print a warning, and taint the kernel" part seems redundant.
Probably there is no need for a comment here as well.

> +	pr_warn("cpu %d: missing required feature(s):", c->cpu_index);
> +	for_each_set_bit(i, (void *)missing, NCAPINTS << 5)

for_each_set_bit() typically expects unsigned long *. Do you run into
any issue if you use that?

> +		pr_cont(" %s", x86_cap_name(i, cap_buf));
> +	pr_cont("\n");
> +	add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
> +}
> +
>  /*
>   * This does the hard work of actually picking apart the CPU stuff...
>   */
> @@ -2125,6 +2156,8 @@ static void identify_cpu(struct cpuinfo_x86 *c)
>  	mcheck_cpu_init(c);
>  
>  	numa_add_cpu(smp_processor_id());
> +
> +	verify_required_features(c);
>  }
>  
>  /*