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

[Maciej Wieczor-Retman]

Thanks for the detailed review!

On 2026-03-10 at 00:05:24 -0700, Sohil Mehta wrote:
>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?

Right, I think that was the previous idea for this function, thanks for spotting
that.

>
>> + */
>> +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.

Sure, I'll clear it up.

>
>> +	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.

Fair enough, I can remove it.

>
>> +	/*
>> +	 * 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.

I would keep the 'At least one required feature is missing' so it's more
readable for someone looking at this for the first time. Looking at it now I
agree the second sentence doesn't help with anything though.

>
>> +	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?


I set some required features to disabled in the x86_capability[] array for a
test and it worked fine. But you're right, it should be a unsigned long *. I'll
change it and retest.

-- 
Kind regards
Maciej Wieczór-Retman