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>

1 files changed, 20 insertions, 0 deletions

diff --git a/arch/x86/kernel/suspend_64.c b/arch/x86/kernel/suspend_64.c
index 2e5efaaf8800..569f1b540e36 100644
--- a/arch/x86/kernel/suspend_64.c
+++ b/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)
 {
         /*