vtime: Update a few comments

Update a stale comment from the old vtime era and document some
locking that might be non obvious.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Kevin Hilman <khilman@linaro.org>

include/linux/context_tracking.h

@@ -72,19 +72,17 @@ extern void guest_exit(void);
 static inline void guest_enter(void)
 {
 	/*
-	 * This is running in ioctl context so we can avoid
-	 * the call to vtime_account() with its unnecessary idle check.
+	 * This is running in ioctl context so its safe
+	 * to assume that it's the stime pending cputime
+	 * to flush.
 	 */
 	vtime_account_system(current);
 	current->flags |= PF_VCPU;
 }
 
 static inline void guest_exit(void)
 {
-	/*
-	 * This is running in ioctl context so we can avoid
-	 * the call to vtime_account() with its unnecessary idle check.
-	 */
+	/* Flush the guest cputime we spent on the guest */
 	vtime_account_system(current);
 	current->flags &= ~PF_VCPU;
 }

kernel/sched/cputime.c

@@ -712,6 +712,13 @@ void vtime_user_enter(struct task_struct *tsk)
 
 void vtime_guest_enter(struct task_struct *tsk)
 {
+	/*
+	 * The flags must be updated under the lock with
+	 * the vtime_snap flush and update.
+	 * That enforces a right ordering and update sequence
+	 * synchronization against the reader (task_gtime())
+	 * that can thus safely catch up with a tickless delta.
+	 */
 	write_seqlock(&tsk->vtime_seqlock);
 	__vtime_account_system(tsk);
 	current->flags |= PF_VCPU;