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

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Message-ID: <xfshxftto4al2syvtrmbqbswlussduvncodyowjwfcvi23v45e@sy4e4hckbf7a>
Date: Wed, 23 Jul 2025 17:13:07 +0200
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: Borislav Petkov <bp@...en8.de>
CC: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	"Dave Hansen" <dave.hansen@...ux.intel.com>, <x86@...nel.org>, "H. Peter
 Anvin" <hpa@...or.com>, "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 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.

>
>As to what the Fixes: tag should be - it should not have any Fixes: tag
>because AFAICT, this has always been this way. So this fix should be
>backported everywhere.

I found that in 5.9-rc1 the documentation for how /proc/cpuinfo should work was
merged [1]. I understand that from that point on, while one can't rely on a
feature's absence, it's a reliable convention that if a flag is present, then
the feature is working. So from 5.9 on, it seems like a bug when these features
show up as working while they're not due to not being compiled.

[1] ea4e3bef4c94 ("Documentation/x86: Add documentation for /proc/cpuinfo feature flags")

>
>Thx.
>
>-- 
>Regards/Gruss,
>    Boris.
>
>https://people.kernel.org/tglx/notes-about-netiquette

-- 
Kind regards
Maciej Wieczór-Retman