x86: hibernation: document __save_processor_state() on x86

Document the fact that __save_processor_state() has to save all CPU
registers referred to by the kernel in case a different kernel is
used to load and restore a hibernation image containing it.
Sigend-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/suspend_64.c

@@ -19,6 +19,21 @@ extern const void __nosave_begin, __nosave_end;
 
 struct saved_context saved_context;
 
+/**
+ *	__save_processor_state - save CPU registers before creating a
+ *		hibernation image and before restoring the memory state from it
+ *	@ctxt - structure to store the registers contents in
+ *
+ *	NOTE: If there is a CPU register the modification of which by the
+ *	boot kernel (ie. the kernel used for loading the hibernation image)
+ *	might affect the operations of the restored target kernel (ie. the one
+ *	saved in the hibernation image), then its contents must be saved by this
+ *	function.  In other words, if kernel A is hibernated and different
+ *	kernel B is used for loading the hibernation image into memory, the
+ *	kernel A's __save_processor_state() function must save all registers
+ *	needed by kernel A, so that it can operate correctly after the resume
+ *	regardless of what kernel B does in the meantime.
+ */
 void __save_processor_state(struct saved_context *ctxt)
 {
 	kernel_fpu_begin();
@@ -69,6 +84,11 @@ static void do_fpu_end(void)
 	kernel_fpu_end();
 }
 
+/**
+ *	__restore_processor_state - restore the contents of CPU registers saved
+ *		by __save_processor_state()
+ *	@ctxt - structure to load the registers contents from
+ */
 void __restore_processor_state(struct saved_context *ctxt)
 {
 	/*

include/asm-x86/suspend_64.h

@@ -15,7 +15,14 @@ arch_prepare_suspend(void)
 	return 0;
 }
 
-/* Image of the saved processor state. If you touch this, fix acpi/wakeup.S. */
+/*
+ * Image of the saved processor state, used by the low level ACPI suspend to
+ * RAM code and by the low level hibernation code.
+ *
+ * If you modify it, fix arch/x86/kernel/acpi/wakeup_64.S and make sure that
+ * __save/__restore_processor_state(), defined in arch/x86/kernel/suspend_64.c,
+ * still work as required.
+ */
 struct saved_context {
 	struct pt_regs regs;
   	u16 ds, es, fs, gs, ss;