linux-kernel - Re: [PATCH v8 018/103] KVM: TDX: Stub in tdx.h with structs, accessors, and VMCS helpers

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Date:   Tue, 23 Aug 2022 15:40:40 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Binbin Wu <binbin.wu@...ux.intel.com>
Cc:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
        Paolo Bonzini <pbonzini@...hat.com>, erdemaktas@...gle.com,
        Sagi Shahar <sagis@...gle.com>
Subject: Re: [PATCH v8 018/103] KVM: TDX: Stub in tdx.h with structs,
 accessors, and VMCS helpers

On Tue, Aug 23, 2022, Binbin Wu wrote:
> 
> On 2022/8/8 6:01, isaku.yamahata@...el.com wrote:
> > +static __always_inline void tdvps_vmcs_check(u32 field, u8 bits)
> > +{
> > +	BUILD_BUG_ON_MSG(__builtin_constant_p(field) && (field) & 0x1,
> > +			 "Read/Write to TD VMCS *_HIGH fields not supported");
> > +
> > +	BUILD_BUG_ON(bits != 16 && bits != 32 && bits != 64);
> > +
> > +	BUILD_BUG_ON_MSG(bits != 64 && __builtin_constant_p(field) &&
> > +			 (((field) & 0x6000) == 0x2000 ||
> > +			  ((field) & 0x6000) == 0x6000),
> > +			 "Invalid TD VMCS access for 64-bit field");
> 
> if bits is 64 here, "bits != 64" is false, how could this check for "Invalid
> TD VMCS access for 64-bit field"?

Bits 14:13 of the encoding, which is extracted by "(field) & 0x6000", encodes the
width of the VMCS field.  Bit 0 of the encoding, "(field) & 0x1" above, is a modifier
that is only relevant when operating in 32-bit mode, and is disallowed because TDX is
64-bit only.

This yields four possibilities for TDX:

  (field) & 0x6000) == 0x0000 : 16-bit field
  (field) & 0x6000) == 0x2000 : 64-bit field
  (field) & 0x6000) == 0x4000 : 32-bit field
  (field) & 0x6000) == 0x6000 : 64-bit field (technically "natural width", but
                                              effectively 64-bit because TDX is
					      64-bit only)

The assertion is that if the encoding indicates a 64-bit field (0x2000 or 0x6000),
then the number of bits KVM is accessing must be '64'.  The below assertions do
the same thing for 32-bit and 16-bit fields.
 
> > +	BUILD_BUG_ON_MSG(bits != 32 && __builtin_constant_p(field) &&
> > +			 ((field) & 0x6000) == 0x4000,
> > +			 "Invalid TD VMCS access for 32-bit field");
> 
> ditto
> 
> 
> > +	BUILD_BUG_ON_MSG(bits != 16 && __builtin_constant_p(field) &&
> > +			 ((field) & 0x6000) == 0x0000,
> > +			 "Invalid TD VMCS access for 16-bit field");
> 
> ditto