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

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Date:   Wed, 13 Oct 2021 22:44:37 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        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>
Cc:     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 Fri, Oct 08 2021 at 22:37, Kuppuswamy Sathyanarayanan wrote:
> +
> +bool is_tdx_guest(void)
> +{
> +	static int tdx_guest = -1;
> +	u32 eax, sig[3];
> +
> +	if (tdx_guest >= 0)
> +		goto done;
> +
> +	if (cpuid_eax(0) < TDX_CPUID_LEAF_ID) {
> +		tdx_guest = 0;
> +		goto done;
> +	}
> +
> +	cpuid_count(TDX_CPUID_LEAF_ID, 0, &eax, &sig[0], &sig[2], &sig[1]);
> +
> +	tdx_guest = !memcmp("IntelTDX    ", sig, 12);
> +
> +done:
> +	return !!tdx_guest;
> +}

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?

> +void __init tdx_early_init(void)
> +{
> +	if (!is_tdx_guest())
> +		return;
> +
> +	setup_force_cpu_cap(X86_FEATURE_TDX_GUEST);
> +
> +	pr_info("Guest initialized\n");
> +}

What's wrong with:

static bool tdx_guest_detected __ro_after_init;

void __init tdx_early_init(void)
{
	u32 eax, sig[3];

	if (cpuid_eax(0) < TDX_CPUID_LEAF_ID)
        	return;

        cpuid_count(TDX_CPUID_LEAF_ID, 0, &eax, &sig[0], &sig[2],  &sig[1]);

        if (memcmp("IntelTDX    ", sig, 12))
        	return;

        tdx_guest_detected = true;
	setup_force_cpu_cap(X86_FEATURE_TDX_GUEST);

	pr_info("Guest initialized\n");
}

bool is_tdx_guest(void)
{
        return tdx_guest_detected;
}

That's simple and obvious and discards all the detection gunk completely
after init and uses the proper data type, right?

Thanks,

        tglx