Re: [PATCH v4] x86/cpufeature: Add feature dependency checks

[Ingo Molnar]

* Sohil Mehta <sohil.mehta@xxxxxxxxx> wrote:

> On 2/27/2025 10:46 AM, Ingo Molnar wrote:
> 
> >> +void filter_feature_dependencies(struct cpuinfo_x86 *c)
> >> +{
> >> +	char feature_buf[16], depends_buf[16];
> >> +	const struct cpuid_dep *d;
> >> +
> >> +	for (d = cpuid_deps; d->feature; d++) {
> >> +		if (cpu_has(c, d->feature) && !cpu_has(c, d->depends)) {
> >> +			pr_info("CPU%d: Disabling feature %s due to missing feature %s\n",
> >> +				smp_processor_id(),
> >> +				x86_feature_name(d->feature, feature_buf),
> >> +				x86_feature_name(d->depends, depends_buf));
> >> +			do_clear_cpu_cap(c, d->feature);
> >> +		}
> >> +	}
> > 
> > So let's not disable any CPU features actively for the time being, how 
> > about issuing a pr_warn() only about the dependency violation?
> > 
> > I think the main problem is when these problems slip through 100% 
> > unnoticed.
> > 
> 
> I guess you are right. Highlighting the issue is the main part. Beyond
> that we can leave the system behavior as-is for now.
> 
> Most of the listed dependencies seem to be spec-driven, though the
> kernel might create arbitrary dependencies for security reasons such as
> making LAM depend on LASS[1]. I think those can probably be handled on a
> case by case basis during specific feature enabling.
> 
> For the new pr_warn(), I am considering printing it only once per
> feature instead of printing it on every CPU (which could be 100s).

Yeah.

> But that would mean tracking it in a global feature_warn bitmap.
> 
> 	DECLARE_BITMAP(feature_warn, MAX_FEATURE_BITS);
> 
> Another option would be run the scan only on the BSP. But that could
> cause some issues to be missed[2].

Just use pr_warn_once().

Yes, this might cause subsequent CPU feature dependency problems to 
stay unreported, but the hope here is that these are rare and get 
fixed, right?

Thanks,

	Ingo