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Date:   Wed, 6 Oct 2021 17:26:33 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Juergen Gross <jgross@suse.com>, Deep Shah <sdeep@vmware.com>,
        VMware Inc <pv-drivers@vmware.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, Peter H Anvin <hpa@zytor.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Sean Christopherson <seanjc@google.com>,
        Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v8 03/11] x86/cpufeatures: Add TDX Guest CPU feature
Message-ID: <YV3AKekdJU0vsbaN@zn.tnic>
References: <20211005025205.1784480-1-sathyanarayanan.kuppuswamy@linux.intel.com>
 <20211005025205.1784480-4-sathyanarayanan.kuppuswamy@linux.intel.com>
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On Mon, Oct 04, 2021 at 07:51:57PM -0700, Kuppuswamy Sathyanarayanan wrote:
> diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c
> new file mode 100644
> index 000000000000..ad3ff5925153
> --- /dev/null
> +++ b/arch/x86/kernel/tdx.c
> @@ -0,0 +1,40 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (C) 2020 Intel Corporation */
> +
> +#undef pr_fmt
> +#define pr_fmt(fmt)     "tdx: " fmt
> +
> +#include <asm/tdx.h>
> +
> +/*
> + * Allocate it in the data region to avoid zeroing it during
> + * BSS initialization. It is mainly used in cc_platform_has()
> + * call during early boot call.
> + */
> +u64 __section(".data") is_tdx_guest = 0;
> +
> +static void __init is_tdx_guest_init(void)
> +{
> +	u32 eax, sig[3];
> +
> +	if (cpuid_eax(0) < TDX_CPUID_LEAF_ID) {
> +		is_tdx_guest = 0;
> +		return;
> +	}
> +
> +	cpuid_count(TDX_CPUID_LEAF_ID, 0, &eax, &sig[0], &sig[2], &sig[1]);
> +
> +	is_tdx_guest = !memcmp("IntelTDX    ", sig, 12);
> +}
> +
> +void __init tdx_early_init(void)
> +{
> +	is_tdx_guest_init();
> +
> +	if (!is_tdx_guest)
> +		return;
> +
> +	setup_force_cpu_cap(X86_FEATURE_TDX_GUEST);
> +
> +	pr_info("Guest initialized\n");
> +}
> -- 

What I meant was this (untested of course).

is_tdx_guest() is the accessor external code queries and you cache the
detected value in tdx_guest so that the one after the first one is
cheap.

/*
 * Allocate it in the data region to avoid zeroing it during
 * BSS initialization. It is mainly used in cc_platform_has()
 * call during early boot call.
 *
 * States whether the kernel is running as a TDX guest.
 */
static int tdx_guest __ro_after_init = -1;

bool is_tdx_guest(void)
{
	u32 eax, sig[3];

	if (tdx_guest >= 0)
		return tdx_guest;

	if (cpuid_eax(0) < TDX_CPUID_LEAF_ID) {
		tdx_guest = 0;
		return false;
	}

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

	tdx_guest = !memcmp("IntelTDX    ", sig, 12);

	return tdx_guest;
}

void __init tdx_early_init(void)
{
	if (!is_tdx_guest())
		return;

	setup_force_cpu_cap(X86_FEATURE_TDX_GUEST);

	pr_info("Guest initialized\n");
}

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette