efi/x86: Avoid redundant cast of EFI firmware service pointer

All EFI firmware call prototypes have been annotated as __efiapi,
permitting us to attach attributes regarding the calling convention
by overriding __efiapi to an architecture specific value.

On 32-bit x86, EFI firmware calls use the plain calling convention
where all arguments are passed via the stack, and cleaned up by the
caller. Let's add this to the __efiapi definition so we no longer
need to cast the function pointers before invoking them.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Matthew Garrett <mjg59@google.com>
Cc: linux-efi@vger.kernel.org
Link: https://lkml.kernel.org/r/20200103113953.9571-6-ardb@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/efi.h

@@ -51,13 +51,7 @@ extern asmlinkage unsigned long efi_call_phys(void *, ...);
 })
 
 
-/*
- * Wrap all the virtual calls in a way that forces the parameters on the stack.
- */
-#define arch_efi_call_virt(p, f, args...)				\
-({									\
-	((efi_##f##_t __attribute__((regparm(0)))*) p->f)(args);	\
-})
+#define arch_efi_call_virt(p, f, args...)	p->f(args)
 
 #define efi_ioremap(addr, size, type, attr)	ioremap_cache(addr, size)
 

include/linux/efi.h

@@ -48,8 +48,10 @@ typedef u16 efi_char16_t;		/* UNICODE character */
 typedef u64 efi_physical_addr_t;
 typedef void *efi_handle_t;
 
-#ifdef CONFIG_X86_64
+#if defined(CONFIG_X86_64)
 #define __efiapi __attribute__((ms_abi))
+#elif defined(CONFIG_X86_32)
+#define __efiapi __attribute__((regparm(0)))
 #else
 #define __efiapi
 #endif