linux-kernel - Re: [PATCH v2] x86/cpufeature: Add feature dependency checks

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Message-ID: <b4fce5bc-833d-452e-bee6-625c88eaf484@intel.com>
Date: Wed, 30 Oct 2024 16:57:33 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: "Luck, Tony" <tony.luck@...el.com>, "x86@...nel.org" <x86@...nel.org>,
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>
CC: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H .
 Peter Anvin" <hpa@...or.com>, Uros Bizjak <ubizjak@...il.com>, Sandipan Das
	<sandipan.das@....com>, Sean Christopherson <seanjc@...gle.com>, "Peter
 Zijlstra" <peterz@...radead.org>, Vegard Nossum <vegard.nossum@...cle.com>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Nikolay Borisov
	<nik.borisov@...e.com>, Eric Biggers <ebiggers@...gle.com>, "Li, Xin3"
	<xin3.li@...el.com>, "Shishkin, Alexander" <alexander.shishkin@...el.com>,
	Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] x86/cpufeature: Add feature dependency checks

On 10/30/2024 4:44 PM, Luck, Tony wrote:
>> +void filter_feature_dependencies(struct cpuinfo_x86 *c)
>> +{
>> +     const struct cpuid_dep *d;
>> +
>> +     for (d = cpuid_deps; d->feature; d++) {
>> +             if (cpu_has(c, d->feature) && !cpu_has(c, d->depends))
>> +                     do_clear_cpu_cap(c, d->feature);
>> +     }
>> +}
> 
> The dependency check found something very wrong. Should there be
> a pr_warn() to give some clue that Linux papered over this problem?
> 

Not sure if this necessitates a warning. A missing feature may not be a
problem for the user. I am treating a disabled feature due to the
original CPUID enumeration not being present vs due to a dependency not
being met as the same.

System and kernel developers would definitely care if an expected
feature is missing. Maybe a pr_debug()? Want to avoid unnecessary
escalations and bug reports.

Sohil