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

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Message-ID: <jaulcyha4kke42um3nnfiw7hvd7kienemqg2breloz6auchqa4@zmzt3his7ovy>
Date: Wed, 23 Jul 2025 18:17:03 +0200
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: "H. Peter Anvin" <hpa@...or.com>
CC: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>, 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 2025-07-23 at 07:08:43 -0700, H. Peter Anvin wrote:
>On 2025-07-23 02:22, Maciej Wieczor-Retman wrote:
>> 
>>  arch/x86/kernel/cpu/common.c       | 12 ++++++++++++
>>  arch/x86/tools/cpufeaturemasks.awk |  8 ++++++++
>>  2 files changed, 20 insertions(+)
>> 
>> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
>> index 77afca95cced..ba8b5fba8552 100644
>> --- a/arch/x86/kernel/cpu/common.c
>> +++ b/arch/x86/kernel/cpu/common.c
>> @@ -1709,6 +1709,16 @@ static void __init cpu_parse_early_param(void)
>>  	}
>>  }
>>  
>> +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);
>> +	}
>> +}
>> +
>
>No... just use an static array initializer for cpu_caps_cleared[].  You
>don't even need to add any code at all.

Oh, right, that seems simpler

>
>Ironically enough, I actually had those macros generated in the original
>awk script version, but they weren't used.
>
>And you MUST NOT initialize cpu_caps_set[]. 

After thinking about it more I can see why doing that could be a problem, yes.

>What we *can* and probably should do is, at the very end of the process, verify
>that the final mask conforms to cpu_caps_set[] and print a nasty message and
>set the TAINT_CPU_OUT_OF_SPEC flag:
>
>	add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK)

Where would that end be though? End of identify_cpu()? apply_forced_caps() is
almost at the end of the function, and other functions don't seem to modify
x86_cpuinfo or cpu_caps_set[].

>... but that is a separate patch, and doesn't need to be backported.
>
>Xin send me a patch privately (attached) but it needs to have the
>REQUIRED_MASK_INIT_VALUES part removed and made into a proper patch.
>
>Xin didn't add an SoB on it, but you can use mine:
>
>Signed-off-by: H. Peter Anvin (Intel) <hpa@...or.com>

Thanks, I'll work with that.

>
>	-hpa



-- 
Kind regards
Maciej Wieczór-Retman