Re: [PATCH] x86/cpufeatures: Free up unused feature bits

[Dave Hansen <dave.hansen@intel.com>]

On 11/7/24 17:12, Dave Hansen wrote:
> and then we recycled number 67:
> 
> -#define X86_FEATURE_P3                  ( 3*32+ 6) /* P3 */
> +#define X86_FEATURE_WHIZZY_NEW_FEATURE  ( 3*32+ 6) /* P3 */
> 
> udev might try to load the old module on a new CPU with
> X86_FEATURE_WHIZZY_NEW_FEATURE that's not a P3.

Thinking about this a bit more...

The kernel generates _both_ the
"cpu:type:x86,ven*fam*mod*:feature:*0067*" string and the sysfs modalias
string.  So the issue isn't practically a mismatch between those.

It's if some consumer of those fields (like /lib/udev/hwdb.d/) was
looking for feature 67.

The good news is that I don't see any of those today.  But it's totally
possible that folks have some crazy rules out there.  So we should
probably be _careful_ about changing those values and not just change
them *ALL*.  But I think it's pretty unlikely we'll break anybody by
reusing a bit or two.