linux-kernel - Re: [PATCHv4 03/30] x86/tdx: Provide common base for SEAMCALL and TDCALL C wrappers

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Date:   Thu, 24 Feb 2022 16:41:12 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        luto@...nel.org, peterz@...radead.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, jpoimboe@...hat.com, knsathya@...nel.org,
        pbonzini@...hat.com, sdeep@...are.com, seanjc@...gle.com,
        tony.luck@...el.com, vkuznets@...hat.com, wanpengli@...cent.com,
        thomas.lendacky@....com, brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 03/30] x86/tdx: Provide common base for SEAMCALL and
 TDCALL C wrappers

On 2/24/22 15:10, Kirill A. Shutemov wrote:
> +/*
> + * SW-defined error codes.
> + *
> + * Bits 47:40 == 0xFF indicate Reserved status code class that never used by
> + * TDX module.
> + */
> +#define TDX_SEAMCALL_VMFAILINVALID     0x8000FF00FFFF0000ULL

That's OK-ish.  But, it would be nice to make this a bit less magic.
While I'm sure plenty of us can do the bits 47:40 => hex math in our
heads, it might be nice to do it with a macro.  Maybe:

/*
 * Bits 47:40 being set represent a reserved status class.
 * The TDX module will never set these so they are safe to
 * use for software error codes.
 */
#define TDX_SW_ERR(code) ((code) | GENMASK_ULL(40, 47))

#define TDX_SEAMCALL_VMFAILINVALID	TDX_SW_ERR(0xFFFF0000ULL)

By the way, is the entire "0xFFFF0000ULL" thing up for grabs?  Or do the
the "0xFFFF...." bits _need_ to be set to represent an error somehow?

Would this work if it were:

#define TDX_SEAMCALL_VMFAILINVALID	TDX_SW_ERR(0ULL)

or

#define TDX_SEAMCALL_VMFAILINVALID	TDX_SW_ERR(1ULL)

or

#define TDX_SEAMCALL_VMFAILINVALID	TDX_SW_ERR(0x12345678ULL)

?