linux-kernel - Re: [RFC PATCH] tools/x86: add kcpuid tool to show raw CPU features

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Date:   Wed, 2 Sep 2020 10:18:38 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Feng Tang <feng.tang@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] tools/x86: add kcpuid tool to show raw CPU features

On 9/2/20 9:52 AM, Borislav Petkov wrote:
>> I was *really* hoping that we could eventually feed kcpuid and the
>> X86_FEATURE_* bits from the same source.
> But X86_FEATURE_* won't be all bits in all CPUID leafs - only the ones the
> kernel has enabled/use for/needs/...
> 
> Also you have CPUID fields which are multi-bit and which cannot be
> X86_FEATURE_* things.
> 
> So you need a place which has the whole CPUID space and X86_FEATURE_*
> simply cannot represent it all.

All good points.

Personally, I think I'd be OK if kcpuid _focused_ on single-bit values
only.  Those are the vast majority of the things that we need from such
a tool.  I don't think there's nearly as much demand for the topology
enumeration or TLB structure leaf parsing.

I also kinda doubt we can come up with a file format off the top of our
heads that can represent all the ways data is represented in the CPUID
space.  Single bits, sure.