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    SubjectRe: [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@intel.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

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