kvm: Note an RCU quiescent state on guest exit

As of today, KVM notes a quiescent state only in guest entry, which is good
as it avoids the guest being interrupted for current RCU operations.

While the guest vcpu runs, it can be interrupted by a timer IRQ that will
check for any RCU operations waiting for this CPU. In case there are any of
such, it invokes rcu_core() in order to sched-out the current thread and
note a quiescent state.

This occasional schedule work will introduce tens of microsseconds of
latency, which is really bad for vcpus running latency-sensitive
applications, such as real-time workloads.

So, note a quiescent state in guest exit, so the interrupted guests is able
to deal with any pending RCU operations before being required to invoke
rcu_core(), and thus avoid the overhead of related scheduler work.
Signed-off-by: Leonardo Bras <leobras@redhat.com>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20240511020557.1198200-1-leobras@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/linux/context_tracking.h

@@ -80,10 +80,12 @@ static __always_inline bool context_tracking_guest_enter(void)
 	return context_tracking_enabled_this_cpu();
 }
 
-static __always_inline void context_tracking_guest_exit(void)
+static __always_inline bool context_tracking_guest_exit(void)
 {
 	if (context_tracking_enabled())
 		__ct_user_exit(CONTEXT_GUEST);
+
+	return context_tracking_enabled_this_cpu();
 }
 
 #define CT_WARN_ON(cond) WARN_ON(context_tracking_enabled() && (cond))
@@ -98,7 +100,7 @@ static inline void exception_exit(enum ctx_state prev_ctx) { }
 static inline int ct_state(void) { return -1; }
 static inline int __ct_state(void) { return -1; }
 static __always_inline bool context_tracking_guest_enter(void) { return false; }
-static __always_inline void context_tracking_guest_exit(void) { }
+static __always_inline bool context_tracking_guest_exit(void) { return false; }
 #define CT_WARN_ON(cond) do { } while (0)
 #endif /* !CONFIG_CONTEXT_TRACKING_USER */
 

include/linux/kvm_host.h

@@ -485,7 +485,15 @@ static __always_inline void guest_state_enter_irqoff(void)
  */
 static __always_inline void guest_context_exit_irqoff(void)
 {
-	context_tracking_guest_exit();
+	/*
+	 * Guest mode is treated as a quiescent state, see
+	 * guest_context_enter_irqoff() for more details.
+	 */
+	if (!context_tracking_guest_exit()) {
+		instrumentation_begin();
+		rcu_virt_note_context_switch();
+		instrumentation_end();
+	}
 }
 
 /*