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SubjectRe: [PATCH v10 03/11] x86/cpufeatures: Add TDX Guest CPU feature
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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

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