kvm: cap halt polling at exactly halt_poll_ns

BugLink: http://bugs.launchpad.net/bugs/1558330

commit 313f636d upstream.

When growing halt-polling, there is no check that the poll time exceeds
the limit. It's possible for vcpu->halt_poll_ns grow once past
halt_poll_ns, and stay there until a halt which takes longer than
vcpu->halt_poll_ns. For example, booting a Linux guest with
halt_poll_ns=11000:

 ... kvm:kvm_halt_poll_ns: vcpu 0: halt_poll_ns 0 (shrink 10000)
 ... kvm:kvm_halt_poll_ns: vcpu 0: halt_poll_ns 10000 (grow 0)
 ... kvm:kvm_halt_poll_ns: vcpu 0: halt_poll_ns 20000 (grow 10000)
Signed-off-by: David Matlack <dmatlack@google.com>
Fixes: aca6ff29Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

virt/kvm/kvm_main.c

@@ -1961,6 +1961,9 @@ static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
 	else
 		val *= halt_poll_ns_grow;
 
+	if (val > halt_poll_ns)
+		val = halt_poll_ns;
+
 	vcpu->halt_poll_ns = val;
 	trace_kvm_halt_poll_ns_grow(vcpu->vcpu_id, val, old);
 }