kvm: cap halt polling at exactly halt_poll_ns

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: aca6ff29c4063a8d467cdee241e6b3bf7dc4a171
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index a11cfd20a6a0d2aa86b1d06b8552bc41a8bfd8c9..9102ae172d2a92e06ad6dc9155985b5103703a92 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1952,6 +1952,9 @@ 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);
 }