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 <[email protected]>
Acked-by: Paul E. McKenney <[email protected]>
Acked-by: Sean Christopherson <[email protected]>
Message-ID: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

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