linux-kernel - Re: [PATCH v10 08/11] x86/tdx: Wire up KVM hypercalls

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Date:   Thu, 14 Oct 2021 20:03:05 -0700
From:   Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        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 08/11] x86/tdx: Wire up KVM hypercalls


On 10/14/21 3:21 AM, Thomas Gleixner wrote:
> On Fri, Oct 08 2021 at 22:37, Kuppuswamy Sathyanarayanan wrote:
>> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
>>   
>>   #ifdef CONFIG_KVM_GUEST
>> @@ -32,6 +34,10 @@ static inline bool kvm_check_and_clear_guest_paused(void)
>>   static inline long kvm_hypercall0(unsigned int nr)
>>   {
>>   	long ret;
>> +
>> +	if (cc_platform_has(CC_ATTR_GUEST_TDX))
>> +		return tdx_kvm_hypercall(nr, 0, 0, 0, 0);
> So if TDX is not enabled in Kconfig this cannot be optimized out unless
> CC_PLATFORM is disabled as well. But what's worse is that every
> hypercall needs to call into cc_platform_has().
>
> None of the hypercalls is used before the early TDX detection. So we can
> simply use
>
>         if (cpu_feature_enabled(X86_FEATURE_TDX_GUEST))
>
> here, right? Then you add X86_FEATURE_TDX_GUEST to the disabled feature
> bits correctly and all of the above is solved.
>
> Hmm?


Make sense. Since this will only be used after tdx_early_init() call,
and X86_FEATURE_TDX_GUEST is also set in that call, we can just use
cpu_feature_enabled(X86_FEATURE_TDX_GUEST) as you have mentioned.

I will fix this in next version.

>   
>> +#if defined(CONFIG_KVM_GUEST) && defined(CONFIG_INTEL_TDX_GUEST)
>> +static inline long tdx_kvm_hypercall(unsigned int nr, unsigned long p1,
>> +				     unsigned long p2, unsigned long p3,
>> +				     unsigned long p4)
>> +{
>> +	struct tdx_hypercall_output out;
>> +	u64 err;
>> +
>> +	err = __tdx_hypercall(TDX_HYPERCALL_VENDOR_KVM, nr, p1, p2,
>> +			      p3, p4, &out);
>> +
>> +	/*
>> +	 * Non zero return value means buggy TDX module (which is fatal).
>> +	 * So use BUG_ON() to panic.
>> +	 */
>> +	BUG_ON(err);
>> +
>> +	return out.r10;
>> +}
> Can we make that a proper exported function (instead of
> tdx_kvm_hypercall) so we don't end up with the very same code inlined
> all over the place?


Initially it was an exported function. But we made it inline in tdx.h
to simplify the implementation. But if exported function is preferred,
I will fix it in next version.

>
> Thanks,
>
>          tglx
>
-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer