x86/virt/tdx: Avoid indirect calls to TDX assembly functions

JIRA: https://issues.redhat.com/browse/RHEL-47242

commit 0b3bc01
Author: Kai Huang <kai.huang@intel.com>
Date:   Sat Jun 7 01:07:37 2025 +1200

    x86/virt/tdx: Avoid indirect calls to TDX assembly functions

    Two 'static inline' TDX helper functions (sc_retry() and
    sc_retry_prerr()) take function pointer arguments which refer to
    assembly functions.  Normally, the compiler inlines the TDX helper,
    realizes that the function pointer targets are completely static --
    thus can be resolved at compile time -- and generates direct call
    instructions.

    But, other times (like when CONFIG_CC_OPTIMIZE_FOR_SIZE=y), the
    compiler declines to inline the helpers and will instead generate
    indirect call instructions.

    Indirect calls to assembly functions require special annotation (for
    various Control Flow Integrity mechanisms).  But TDX assembly
    functions lack the special annotations and can only be called
    directly.

    Annotate both the helpers as '__always_inline' to prod the compiler
    into maintaining the direct calls. There is no guarantee here, but
    Peter has volunteered to report the compiler bug if this assumption
    ever breaks[1].

    Fixes: 1e66a7e ("x86/virt/tdx: Handle SEAMCALL no entropy error in common code")
    Fixes: df01f5a ("x86/virt/tdx: Add SEAMCALL error printing for module initialization")
    Signed-off-by: Kai Huang <kai.huang@intel.com>
    Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
    Cc: stable@vger.kernel.org
    Link: https://lore.kernel.org/lkml/20250605145914.GW39944@noisy.programming.kicks-ass.net/ [1]
    Link: https://lore.kernel.org/all/20250606130737.30713-1-kai.huang%40intel.com

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>

Author: Maxim Levitsky

arch/x86/include/asm/tdx.h

@@ -101,7 +101,7 @@ void tdx_init(void);
 
 typedef u64 (*sc_func_t)(u64 fn, struct tdx_module_args *args);
 
-static inline u64 sc_retry(sc_func_t func, u64 fn,
+static __always_inline u64 sc_retry(sc_func_t func, u64 fn,
 			   struct tdx_module_args *args)
 {
 	int retry = RDRAND_RETRY_LOOPS;

arch/x86/virt/vmx/tdx/tdx.c

@@ -75,8 +75,9 @@ static inline void seamcall_err_ret(u64 fn, u64 err,
 			args->r9, args->r10, args->r11);
 }
 
-static inline int sc_retry_prerr(sc_func_t func, sc_err_func_t err_func,
-				 u64 fn, struct tdx_module_args *args)
+static __always_inline int sc_retry_prerr(sc_func_t func,
+					  sc_err_func_t err_func,
+					  u64 fn, struct tdx_module_args *args)
 {
 	u64 sret = sc_retry(func, fn, args);