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

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Message-ID: <877deg8vn4.ffs@tglx>
Date:   Wed, 13 Oct 2021 23:25:35 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     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 23:07, Borislav Petkov wrote:
> On Wed, Oct 13, 2021 at 10:44:37PM +0200, Thomas Gleixner wrote:
>> No. This is tasteless garbage, really.
>> 
>> tdx_early_init() is invoked from x86_64_start_kernel() very early in the
>> boot process __before__ is_tdx_guest() is invoked.
>> 
>> So why on earth is it requried to keep those conditionals and cpuid()
>> muck around after init?
>
> Yah, reportedly, they wanna parse cmdline options so it has to be after
> copy_bootdata() but copy_bootdata() has a cc_platform_has() call which
> ends up in is_tdx_guest() on Intel and there you have the catch 22
> because CPUID hasn't happened yet in tdx_early_init().

Seriously?

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.

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.

Thanks,

        tglx