Re: [PATCH v11 3/4] x86/cpu: Do a sanity check on required feature bits

[Pawan Gupta]

On Fri, Mar 20, 2026 at 12:50:25PM +0000, Maciej Wieczor-Retman wrote:
...
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 0e318f3d56cb..92159a0963c8 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -2005,6 +2005,38 @@ const char *x86_cap_name(unsigned int bit, char *buf)
>  	return buf;
>  }
>  
> +/*
> + * As a sanity check compare the final x86_capability bitmask with the initial
> + * predefined required feature bits.
> + */
> +static void verify_required_features(const struct cpuinfo_x86 *c)
> +{
> +	u32 required_features[NCAPINTS] = REQUIRED_MASK_INIT;
> +	DECLARE_BITMAP(missing, NCAPINTS * 32);
> +	char cap_buf[X86_CAP_BUF_SIZE];
> +	u32 *missing_u32;
> +	unsigned int i;
> +	u32 error = 0;
> +
> +	memset(missing, 0, sizeof(missing));
> +	missing_u32 = (u32 *)missing;
> +
> +	for (i = 0; i < NCAPINTS; i++) {
> +		missing_u32[i] = ~c->x86_capability[i] & required_features[i];
> +		error |= missing_u32[i];
> +	}
> +
> +	if (!error)
> +		return;
> +
> +	/* At least one required feature is missing */
> +	pr_warn("CPU %d: missing required feature(s):", c->cpu_index);
> +	for_each_set_bit(i, missing, NCAPINTS * 32)
> +		pr_cont(" %s", x86_cap_name(i, cap_buf));
> +	pr_cont("\n");
> +	add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
> +}

Do we need 2 loops? Can this be simplified as below:

static void verify_required_features(const struct cpuinfo_x86 *c)
{
	u32 required_features[NCAPINTS + 1] = REQUIRED_MASK_INIT;
	char cap_buf[X86_CAP_BUF_SIZE];
	int i, error = 0;

	for_each_set_bit(i, (unsigned long *)required_features, NCAPINTS * 32) {
		if (test_bit(i, (unsigned long *)c->x86_capability))
			continue;
		if (!error)
			pr_warn("CPU %d: missing required feature(s):", c->cpu_index);
		pr_cont(" %s", x86_cap_name(i, cap_buf));
		error = 1;
	}

	if (!error)
		return;

	pr_cont("\n");
	add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
}