kvm,rcu,nohz: use RCU extended quiescent state when running KVM guest

The host kernel is not doing anything while the CPU is executing
a KVM guest VCPU, so it can be marked as being in an extended
quiescent state, identical to that used when running user space
code.

The only exception to that rule is when the host handles an
interrupt, which is already handled by the irq code, which
calls rcu_irq_enter and rcu_irq_exit.

The guest_enter and guest_exit functions already switch vtime
accounting independent of context tracking. Leave those calls
where they are, instead of moving them into the context tracking
code.

Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Rik van Riel <riel@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will deacon <will.deacon@arm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>

include/linux/context_tracking.h

@@ -81,10 +81,16 @@ static inline void guest_enter(void)
 		vtime_guest_enter(current);
 	else
 		current->flags |= PF_VCPU;
+
+	if (context_tracking_is_enabled())
+		context_tracking_enter(CONTEXT_GUEST);
 }
 
 static inline void guest_exit(void)
 {
+	if (context_tracking_is_enabled())
+		context_tracking_exit(CONTEXT_GUEST);
+
 	if (vtime_accounting_enabled())
 		vtime_guest_exit(current);
 	else

include/linux/context_tracking_state.h

@@ -15,6 +15,7 @@ struct context_tracking {
 	enum ctx_state {
 		CONTEXT_KERNEL = 0,
 		CONTEXT_USER,
+		CONTEXT_GUEST,
 	} state;
 };
 

include/linux/kvm_host.h

@@ -766,7 +766,8 @@ static inline void kvm_guest_enter(void)
 	 * one time slice). Lets treat guest mode as quiescent state, just like
 	 * we do with user-mode execution.
 	 */
-	rcu_virt_note_context_switch(smp_processor_id());
+	if (!context_tracking_cpu_is_enabled())
+		rcu_virt_note_context_switch(smp_processor_id());
 }
 
 static inline void kvm_guest_exit(void)