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

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Message-ID: <84E58634-C57A-4841-BD52-2E15BD9DF592@zytor.com>
Date: Wed, 30 Oct 2024 18:59:02 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: "Luck, Tony" <tony.luck@...el.com>, "Mehta, Sohil" <sohil.mehta@...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>,
        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 October 30, 2024 4:44:59 PM PDT, "Luck, Tony" <tony.luck@...el.com> 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?
>
>-Tony
>

Not necessarily. Linux is free to impose restrictions that don't necessarily match the hardware thermometers. For example, letting LAM depend on LASS.