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Subject: Re: [PATCH v8 02/11] x86/tdx: Introduce INTEL_TDX_GUEST config option
To:     Sathyanarayanan Kuppuswamy Natarajan 
        <sathyanarayanan.nkuppuswamy@gmail.com>,
        Randy Dunlap <rdunlap@infradead.org>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
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        VMware Inc <pv-drivers@vmware.com>,
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        Jim Mattson <jmattson@google.com>,
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        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Sean Christopherson <seanjc@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Message-ID: <8a484fb1-21c5-5181-3e77-c818bcc2ffa7@intel.com>
Date:   Tue, 5 Oct 2021 07:09:59 -0700
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On 10/5/21 6:29 AM, Sathyanarayanan Kuppuswamy Natarajan wrote:
> On Mon, Oct 4, 2021 at 9:53 PM Randy Dunlap <rdunlap@infradead.org> wrote:
>> On 10/4/21 7:51 PM, Kuppuswamy Sathyanarayanan wrote:
>>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>>> index 2b2a9639d8ae..c42dd8a2d1f4 100644
>>> --- a/arch/x86/Kconfig
>>> +++ b/arch/x86/Kconfig
>>> @@ -865,6 +865,20 @@ config ACRN_GUEST
>>>         IOT with small footprint and real-time features. More details can be
>>>         found inhttps://projectacrn.org/.
>>>
>>> +config INTEL_TDX_GUEST
>>> +     bool "Intel Trusted Domain eXtensions Guest Support"
>>> +     depends on X86_64 && CPU_SUP_INTEL && PARAVIRT
>>> +     depends on SECURITY
>>> +     select X86_X2APIC
>> Apparently some Intel CPUs don't have the x2apic feature, since the
>> Kconfig help text for X86_X2APIC says:
>>
>>           This enables x2apic support on CPUs that have this feature.
>>
>> so how is it safe to set/enable/select that kconfig symbol?
>>
>> Will the x2apic code just safely not work if the h/w feature is
>> missing?
> For the TDX guest, x2apic will be emulated. So it will exist in our
> case. Even if x2apic or TDX guest is not supported by CPU, it will
> boot just fine.

This doesn't really explain the "select X86_X2APIC", though.

You just said that TDX doesn't *require* X2APIC.  So, why is it being
selected?  What is the specific connection between TDX and X2APIC?