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

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Date:   Thu, 25 Aug 2022 21:48:17 -0700
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Binbin Wu <binbin.wu@...ux.intel.com>, 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 at 03:40:40PM +0000,
Sean Christopherson <seanjc@...gle.com> wrote:

> 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.

Thanks for explanation. I've updated it as follows to use symbolic value.

#define VMCS_ENC_ACCESS_TYPE_MASK	0x1UL
#define VMCS_ENC_ACCESS_TYPE_FULL	0x0UL
#define VMCS_ENC_ACCESS_TYPE_HIGH	0x1UL
#define VMCS_ENC_ACCESS_TYPE(field)	((field) & VMCS_ENC_ACCESS_TYPE_MASK)

	/* TDX is 64bit only.  HIGH field isn't supported. */
	BUILD_BUG_ON_MSG(__builtin_constant_p(field) &&
			 VMCS_ENC_ACCESS_TYPE(field) == VMCS_ENC_ACCESS_TYPE_HIGH,
			 "Read/Write to TD VMCS *_HIGH fields not supported");

	BUILD_BUG_ON(bits != 16 && bits != 32 && bits != 64);

#define VMCS_ENC_WIDTH_MASK	GENMASK_UL(14, 13)
#define VMCS_ENC_WIDTH_16BIT	(0UL << 13)
#define VMCS_ENC_WIDTH_64BIT	(1UL << 13)
#define VMCS_ENC_WIDTH_32BIT	(2UL << 13)
#define VMCS_ENC_WIDTH_NATURAL	(3UL << 13)
#define VMCS_ENC_WIDTH(field)	((field) & VMCS_ENC_WIDTH_MASK)

	/* TDX is 64bit only.  i.e. natural width = 64bit. */
	BUILD_BUG_ON_MSG(bits != 64 && __builtin_constant_p(field) &&
			 (VMCS_ENC_WIDTH(field) == VMCS_ENC_WIDTH_64BIT ||
			  VMCS_ENC_WIDTH(field) == VMCS_ENC_WIDTH_NATURAL),
			 "Invalid TD VMCS access for 64-bit field");
	BUILD_BUG_ON_MSG(bits != 32 && __builtin_constant_p(field) &&
			 VMCS_ENC_WIDTH(field) == VMCS_ENC_WIDTH_32BIT,
			 "Invalid TD VMCS access for 32-bit field");
	BUILD_BUG_ON_MSG(bits != 16 && __builtin_constant_p(field) &&
			 VMCS_ENC_WIDTH(field) == VMCS_ENC_WIDTH_16BIT,
			 "Invalid TD VMCS access for 16-bit field");

-- 
Isaku Yamahata <isaku.yamahata@...il.com>