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

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

On 8/22/2024 4:27 PM, Sean Christopherson wrote:
> On Thu, Aug 22, 2024, Sohil Mehta wrote:
>> Arguably, this situation should only happen on broken hardware and it may not
>> make sense to add such a check to the kernel. OTOH, this can be viewed as a
>> safety mechanism to make failures more graceful on such configurations in real
>> or virtual environments.
> 
> And goofy Kconfigs.   But yeah, lack of any meaningful fallout is why my version
> didn't go anywhere.
> 

By fallout do you mean that the observed behavior when the kernel runs
into such a misconfiguration or just the general lack of such
misconfigured hardware/guest?

I tried experimenting with the behavior for the last entry on the
cpuid_deps[] table:
{ X86_FEATURE_FRED,                     X86_FEATURE_WRMSRNS   },

In this case, even if WRMSRNS is not present, the kernel would go ahead
and enable FRED, which would cause a panic when wrmsrns() is exercised
in update_task_stack().

I agree to the second part that such conditions are more likely to
happen in pre-production environments. But I still feel that for the
rare case when something like this seeps through it would be better to
disable the feature upfront than run in a kernel panic or some other
unexpected behavior.

> https://lore.kernel.org/all/20221203003745.1475584-2-seanjc@google.com
> 

The code is very similar to the one I proposed. If we do take this
forward, would it be fine if I add a Originally-by tag from you?


>> +void filter_feature_dependencies(struct cpuinfo_x86 *c)
>> +{
>> +	const struct cpuid_dep *d;
>> +
>> +	for (d = cpuid_deps; d->feature; d++) {
>> +		if (boot_cpu_has(d->feature) && !boot_cpu_has(d->depends))
> 
> I don't think checking boot_cpu_has() is correct, it's entirely possible for a CPU
> to have divergent features from the boot CPU, e.g. if a feature is dependent on
> BIOS enabling (or disabling) and BIOS messed up.
> 

Yeah, makes sense. cpu_has() would be better suited as you have done in
your original patch.

>> +			do_clear_cpu_cap(c, d->feature);
>> +	}
>> +}
>> -- 
>> 2.34.1
>>