x86: Extend #DF debugging aid to 64-bit

It is sometimes very helpful to be able to pinpoint the location which
causes a double fault before it turns into a triple fault and the
machine reboots. We have this for 32-bit already so extend it to 64-bit.
On 64-bit we get the register snapshot at #DF time and not from the
first exception which actually causes the #DF. It should be close
enough, though.

[ hpa: and definitely better than nothing, which is what we have now. ]

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1368093749-31296-1-git-send-email-bp@alien8.de
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/Kconfig.debug

@@ -122,7 +122,6 @@ config DEBUG_NX_TEST
 config DOUBLEFAULT
 	default y
 	bool "Enable doublefault exception handler" if EXPERT
-	depends on 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

@@ -981,5 +981,5 @@ bool xen_set_default_idle(void);
 #endif
 
 void stop_this_cpu(void *dummy);
-
+void df_debug(struct pt_regs *regs, long error_code);
 #endif /* _ASM_X86_PROCESSOR_H */

arch/x86/kernel/Makefile

@@ -67,7 +67,7 @@ obj-$(CONFIG_KEXEC)		+= relocate_kernel_$(BITS).o crash.o
 obj-$(CONFIG_CRASH_DUMP)	+= crash_dump_$(BITS).o
 obj-y				+= kprobes/
 obj-$(CONFIG_MODULES)		+= module.o
-obj-$(CONFIG_DOUBLEFAULT) 	+= doublefault_32.o
+obj-$(CONFIG_DOUBLEFAULT)	+= doublefault.o
 obj-$(CONFIG_KGDB)		+= kgdb.o
 obj-$(CONFIG_VM86)		+= vm86_32.o
 obj-$(CONFIG_EARLY_PRINTK)	+= early_printk.o

arch/x86/kernel/doublefault.c

@@ -9,6 +9,8 @@
 #include <asm/processor.h>
 #include <asm/desc.h>
 
+#ifdef CONFIG_X86_32
+
 #define DOUBLEFAULT_STACKSIZE (1024)
 static unsigned long doublefault_stack[DOUBLEFAULT_STACKSIZE];
 #define STACK_START (unsigned long)(doublefault_stack+DOUBLEFAULT_STACKSIZE)
@@ -67,3 +69,16 @@ struct tss_struct 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

@@ -254,6 +254,9 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
 	tsk->thread.error_code = error_code;
 	tsk->thread.trap_nr = X86_TRAP_DF;
 
+#ifdef CONFIG_DOUBLEFAULT
+	df_debug(regs, error_code);
+#endif
 	/*
 	 * This is always a kernel trap and never fixable (and thus must
 	 * never return).