Re: [PATCH 17/20] x86/tdx: Convert VM_RD/VM_WR tdcalls to use new TDCALL macros

[Dave Hansen <dave.hansen@intel.com>]

Let's say you're debugging tdg_vm_rd().  You suspect someone read the
spec wrong.  You pull up the spec:

	https://sr71.net/~dave/intel/tdg.vm.rd.png

On 5/17/24 07:19, Kirill A. Shutemov wrote:
>  static inline u64 tdg_vm_rd(u64 field, u64 *value)
>  {
> -	struct tdx_module_args args = {
> -		.rdx = field,
> -	};

RDX is assigned 'field'.  Makes sense based on the input operands.

> -	u64 ret;
> -
> -	ret = __tdcall_ret(TDG_VM_RD, &args)> -	*value = args.r8;

'value' is set to r8.  Also matches the spec.  It's obvious that this is
a 'two return values' pattern.

> -	return ret;

This is also obviously correct.

Compare that to:

> +	return TDCALL_1(TDG_VM_RD, 0, field, 0, 0, value);
>  }

Where it's 100% opaque which registers thing to into or that 'value' is
an output, not an input.

So, yeah, this is fewer lines of C code.  But it's *WAY* less
self-documenting.  It's harder to audit.  It's harder to understand and
it's more opaque.

While the goals here are laudable, I'm not a big fan of the end result.