linux-kernel - Re: [PATCH] x86/cpufeatures: Free up unused feature bits

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Message-ID: <74338393-4F39-4A04-83B1-BA64CE2D2FDA@zytor.com>
Date: Fri, 08 Nov 2024 00:49:22 +0100
From: "H. Peter Anvin" <hpa@...or.com>
To: Sohil Mehta <sohil.mehta@...el.com>, Dave Hansen <dave.hansen@...el.com>,
        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>,
        Sean Christopherson <seanjc@...gle.com>,
        Tony Luck <tony.luck@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/cpufeatures: Free up unused feature bits

On November 8, 2024 12:44:57 AM GMT+01:00, Sohil Mehta <sohil.mehta@...el.com> wrote:
>On 11/7/2024 3:36 PM, Dave Hansen wrote:
>> On 11/7/24 15:30, Sohil Mehta wrote:
>>> Linux defined feature bits X86_FEATURE_P3 and X86_FEATURE_P4 are not
>>> used anywhere, neither are they visible to userspace.
>>> commit f31d731e4467 ("x86: use X86_FEATURE_NOPL in alternatives") got
>>> rid of the last usage. Remove the related mappings and code.
>> 
>> Hah, not referenced since 2008!  This one seems like a no-brainer.
>
>Thankfully, it wasn't referenced anywhere. For a couple of minutes I was
>wondering why all family 6 CPUs are marked as Pentium 3 on 32-bit.
>
>> @@ -628,11 +628,6 @@ static void init_intel(struct cpuinfo_x86 *c)
>>  		if (p)
>>  			strcpy(c->x86_model_id, p);
>>  	}
>
>Here..
>
>> -	if (c->x86 == 6)
>> -		set_cpu_cap(c, X86_FEATURE_P3);
>

Be careful - these bits are used in module strings and so modutils need to understand them.