x86/tdx: Fix __noreturn build warning around __tdx_hypercall_failed()

LKP reported below build warning:

vmlinux.o: warning: objtool: __tdx_hypercall+0x128: __tdx_hypercall_failed() is missing a __noreturn annotation

The __tdx_hypercall_failed() function definition already has __noreturn
annotation, but it turns out the __noreturn must be annotated to the
function declaration.

PeterZ explains:

"FWIW, the reason being that...

The point of noreturn is that the caller should know to stop generating
code. For that the declaration needs the attribute, because call sites
typically do not have access to the function definition in C."

Add __noreturn annotation to the declaration of __tdx_hypercall_failed()
to fix. It's not a bad idea to document the __noreturn nature at the
definition site either, so keep the annotation at the definition.

Note <asm/shared/tdx.h> is also included by TDX related assembly files.
Include <linux/compiler_attributes.h> only in case of !__ASSEMBLY__
otherwise compiling assembly file would trigger build error.

Also, following the objtool documentation, add __tdx_hypercall_failed()
to "tools/objtool/noreturns.h".

Fixes: c641cfb5c157 ("x86/tdx: Make TDX_HYPERCALL asm similar to TDX_MODULE_CALL")
Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Kai Huang <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/

arch/x86/include/asm/shared/tdx.h

···
 
 #ifndef __ASSEMBLY__
 
+#include <linux/compiler_attributes.h>
+
 /*
  * Used in __tdcall*() to gather the input/output registers' values of the
  * TDCALL instruction when requesting services from the TDX module. This is a
···
 
 
 /* Called from __tdx_hypercall() for unrecoverable failure */
-void __tdx_hypercall_failed(void);
+void __noreturn __tdx_hypercall_failed(void);
 
 bool tdx_accept_memory(phys_addr_t start, phys_addr_t end);
 

tools/objtool/noreturns.h

···
 NORETURN(__module_put_and_kthread_exit)
 NORETURN(__reiserfs_panic)
 NORETURN(__stack_chk_fail)
+NORETURN(__tdx_hypercall_failed)
 NORETURN(__ubsan_handle_builtin_unreachable)
 NORETURN(arch_call_rest_init)
 NORETURN(arch_cpu_idle_dead)