x86/traps: Disentangle the 32-bit and 64-bit doublefault code

The 64-bit doublefault handler is much nicer than the 32-bit one.
As a first step toward unifying them, make the 64-bit handler
self-contained.  This should have no effect no functional effect
except in the odd case of x86_64 with CONFIG_DOUBLEFAULT=n in which
case it will change the logging a bit.

This also gets rid of CONFIG_DOUBLEFAULT configurability on 64-bit
kernels.  It didn't do anything useful -- CONFIG_DOUBLEFAULT=n
didn't actually disable doublefault handling on x86_64.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/Kconfig.debug

@@ -117,7 +117,7 @@ config DEBUG_WX
 
 config DOUBLEFAULT
 	default y
-	bool "Enable doublefault exception handler" if EXPERT
+	bool "Enable doublefault exception handler" if EXPERT && X86_32
 	---help---
 	  This option allows trapping of rare doublefault exceptions that
 	  would otherwise cause a system to silently reboot. Disabling this

arch/x86/include/asm/processor.h

@@ -997,7 +997,6 @@ bool xen_set_default_idle(void);
 #endif
 
 void stop_this_cpu(void *dummy);
-void df_debug(struct pt_regs *regs, long error_code);
 void microcode_check(void);
 
 enum l1tf_mitigations {

arch/x86/kernel/doublefault.c

@@ -72,15 +72,4 @@ struct x86_hw_tss doublefault_tss __cacheline_aligned = {
 	.__cr3		= __pa_nodebug(swapper_pg_dir),
 };
 
-/* dummy for do_double_fault() call */
-void df_debug(struct pt_regs *regs, long error_code) {}
-
-#else /* !CONFIG_X86_32 */
-
-void df_debug(struct pt_regs *regs, long error_code)
-{
-	pr_emerg("PANIC: double fault, error_code: 0x%lx\n", error_code);
-	show_regs(regs);
-	panic("Machine halted.");
-}
 #endif

arch/x86/kernel/traps.c

@@ -411,15 +411,9 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code, unsign
 		handle_stack_overflow("kernel stack overflow (double-fault)", regs, cr2);
 #endif
 
-#ifdef CONFIG_DOUBLEFAULT
-	df_debug(regs, error_code);
-#endif
-	/*
-	 * This is always a kernel trap and never fixable (and thus must
-	 * never return).
-	 */
-	for (;;)
-		die(str, regs, error_code);
+	pr_emerg("PANIC: double fault, error_code: 0x%lx\n", error_code);
+	show_regs(regs);
+	panic("Machine halted.");
 }
 #endif