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

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Date:   Wed, 13 Oct 2021 14:05:42 -0700
From:   Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuwamy@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        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 10/13/21 1:44 PM, Thomas Gleixner wrote:
> 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?

is_tdx_guest() is also used by cc_platform_has() API. Please check
the following patch [1]. cc_platform_has() will be called much earlier
than x86_64_start_kernel() (like __startup_64() [2]).

[1] - 
https://lore.kernel.org/lkml/20211009053747.1694419-5-sathyanarayanan.kuppuswamy@linux.intel.com/
[2] - 
https://lore.kernel.org/lkml/46a18427dc4e9dda985b10e472965e3e4c769f1d.1631141919.git.thomas.lendacky@amd.com/

>
>> +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

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer