x86/fred: Let ret_from_fork_asm() jmp to asm_fred_exit_user when FRED is enabled

Let ret_from_fork_asm() jmp to asm_fred_exit_user when FRED is enabled,
otherwise the existing IDT code is chosen.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Xin Li <xin3.li@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Shan Kang <shan.kang@intel.com>
Link: https://lore.kernel.org/r/20231205105030.8698-29-xin3.li@intel.com

Author: H. Peter Anvin (Intel)

arch/x86/entry/entry_64.S

@@ -247,7 +247,13 @@ SYM_CODE_START(ret_from_fork_asm)
 	 * and unwind should work normally.
 	 */
 	UNWIND_HINT_REGS
+
+#ifdef CONFIG_X86_FRED
+	ALTERNATIVE "jmp swapgs_restore_regs_and_return_to_usermode", \
+		    "jmp asm_fred_exit_user", X86_FEATURE_FRED
+#else
 	jmp	swapgs_restore_regs_and_return_to_usermode
+#endif
 SYM_CODE_END(ret_from_fork_asm)
 .popsection
 

arch/x86/entry/entry_64_fred.S

@@ -32,6 +32,7 @@
 SYM_CODE_START_NOALIGN(asm_fred_entrypoint_user)
 	FRED_ENTER
 	call	fred_entry_from_user
+SYM_INNER_LABEL(asm_fred_exit_user, SYM_L_GLOBAL)
 	FRED_EXIT
 	ERETU
 SYM_CODE_END(asm_fred_entrypoint_user)