linux-kernel - Re: [PATCH v10 03/11] x86/cpufeatures: Add TDX Guest CPU feature

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Message-ID: <87zgrc7clg.ffs@tglx>
Date:   Thu, 14 Oct 2021 01:02:19 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuwamy@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>, Deep Shah <sdeep@...are.com>,
        VMware Inc <pv-drivers@...are.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, Peter H Anvin <hpa@...or.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 03/11] x86/cpufeatures: Add TDX Guest CPU feature

On Wed, Oct 13 2021 at 15:28, Sathyanarayanan Kuppuswamy wrote:
> On 10/13/21 2:37 PM, Borislav Petkov wrote:
>> On Wed, Oct 13, 2021 at 11:25:35PM +0200, Thomas Gleixner wrote:
>>> So this ends up in doing:
>>>
>>>     use();
>>>     init();
>>>
>>> Can you spot what's wrong with that?
>>>
>>> That's a clear violation of common sense and is simply not going to
>>> happen. Why? If you think about deep defensive programming then use()
>>> will look like this:
>>>
>>> use()
>>> {
>>>          assert(initialized);
>>> }
>>>
>>> which is not something made up. It's a fundamental principle of
>>> programming and some languages enforce that for very good reasons.
>>>
>>> Just because it can be done in C is no justification.
>> Oh, I heartily agree.
>>   
>>> What's wrong with:
>>>
>>> x86_64_start_kernel()
>>>
>>>      tdx_early_init();
>>>
>>>      copy_bootdata();
>>>      
>>>      tdx_late_init();
>>>
>>> Absolutely nothing. It's clear, simple and well defined.
>> I like simple more than anyone, so sure, I'd prefer that a lot more.
>>
>> And so the options parsing would need to happen early using, say,
>> cmdline_find_option() or so, like sme_enable() does.
>
> Since in tdx_early_init() all we are going to do is to initialize
> "tdx_guest_detected" using cpuid call, shall we name it
> tdx_guest_cpuid_init()? (similar to sme_enable call in AMD)

How is that similar?

Just chose a name which makes sense in the overall scheme. I surely care
about naming convetions, but what I care more about is correctness.

Whether it ends up being named

        tdx_enable() - to match the SME muck

or

        tdx_detect()

or whatever makes sense does not really matter. As long as it makes
sense. That's bikeshed painting realm.

Coming back to your suggestion 'tdx_guest_cpuid_init()'. Just sit back
and think about what that name says:

    tdx_guest_cpuid_init()

For the uniformed reader this says:

    If tdx guest then initialize CPUID

which is obviously not what you want to express, right?

So, naming matters but you are free to chose something which makes
sense.

Thanks,

        tglx