Use newly introduced TDCALL instead of tdcall() to issue VM_RD/VM_WR
tdcalls

It increase code slightly:

Function                                     old     new   delta
tdx_early_init                               744     776     +32

but combined with VP_INFO changes the total effect on tdx_early_init()
is code reduction.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 arch/x86/coco/tdx/tdx.c | 18 ++----------------
 1 file changed, 2 insertions(+), 16 deletions(-)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index e1849878f3bc..6559f3842f67 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -76,27 +76,13 @@ static inline void tdcall(u64 fn, struct tdx_module_args *args)
 /* Read TD-scoped metadata */
 static inline u64 tdg_vm_rd(u64 field, u64 *value)
 {
-	struct tdx_module_args args = {
-		.rdx = field,
-	};
-	u64 ret;
-
-	ret = __tdcall_ret(TDG_VM_RD, &args);
-	*value = args.r8;
-
-	return ret;
+	return TDCALL_1(TDG_VM_RD, 0, field, 0, 0, value);
 }
 
 /* Write TD-scoped metadata */
 static inline u64 tdg_vm_wr(u64 field, u64 value, u64 mask)
 {
-	struct tdx_module_args args = {
-		.rdx = field,
-		.r8 = value,
-		.r9 = mask,
-	};
-
-	return __tdcall(TDG_VM_WR, &args);
+	return TDCALL_1(TDG_VM_WR, 0, field, value, mask, value);
 }
 
 /**
-- 
2.43.0

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.