linux-kernel - Re: [PATCH v2] x86: Clear feature bits disabled at compile-time

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Message-ID: <6730DE37-744C-44BE-BB51-CF41D87EBBE9@zytor.com>
Date: Wed, 23 Jul 2025 10:23:15 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
CC: Borislav Petkov <bp@...en8.de>, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "Kirill A. Shutemov" <kas@...nel.org>,
        Alexander Potapenko <glider@...gle.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Xin Li <xin3.li@...el.com>,
        Sai Praneeth <sai.praneeth.prakhya@...el.com>,
        Jethro Beekman <jethro@...tanix.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghua.yu@...el.com>,
        "Mike Rapoport (IBM)" <rppt@...nel.org>, Kees Cook <kees@...nel.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>, stable@...r.kernel.org,
        Borislav Petkov <bp@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86: Clear feature bits disabled at compile-time

On July 23, 2025 10:13:52 AM PDT, Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com> wrote:
>On 2025-07-23 at 08:28:32 -0700, H. Peter Anvin wrote:
>>On July 23, 2025 8:13:07 AM PDT, Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com> wrote:
>>>On 2025-07-23 at 15:46:40 +0200, Borislav Petkov wrote:
>>>>On Wed, Jul 23, 2025 at 11:22:49AM +0200, Maciej Wieczor-Retman wrote:
>>>>> +static __init void init_cpu_cap(struct cpuinfo_x86 *c)
>>>>> +{
>>>>> +	int i;
>>>>> +
>>>>> +	for (i = 0; i < NCAPINTS; i++) {
>>>>> +		cpu_caps_set[i] = REQUIRED_MASK(i);
>>>>> +		cpu_caps_cleared[i] = DISABLED_MASK(i);
>>>>> +	}
>>>>> +}
>>>>
>>>>There's already apply_forced_caps(). Not another cap massaging function
>>>>please. Add that stuff there.
>>>
>>>I'll try that, but can't it overwrite some things? apply_forced_caps() is called
>>>three times and cpu_caps_set/cleared are modified in between from what I can
>>>see. init_cpu_cap() was supposed to only initialize these arrays.
>>>
>>What are you concerned it would overwrite? I'm confused.
>
>I thought that cpu_caps_set/cleared could change in-between apply_forced_caps()
>calls. Therefore if we also applied the DISABLED_MASK() in every
>apply_forced_caps() call I thought it might clear some flag that other function
>might set.
>
>But I've been looking at these calls for a while now and that doesn't seem
>possible. Changes are made only if features are compiled, so it doesn't
>interfere with the DISABLED_MASK().
>
>Sorry for the confusion.
>

Any changes would be additive, or we would be in a world of hurt anyway.