MIPS: KVM: Fix timer IRQ race when freezing timer

There's a particularly narrow and subtle race condition when the
software emulated guest timer is frozen which can allow a guest timer
interrupt to be missed.

This happens due to the hrtimer expiry being inexact, so very
occasionally the freeze time will be after the moment when the emulated
CP0_Count transitions to the same value as CP0_Compare (so an IRQ should
be generated), but before the moment when the hrtimer is due to expire
(so no IRQ is generated). The IRQ won't be generated when the timer is
resumed either, since the resume CP0_Count will already match CP0_Compare.

With VZ guests in particular this is far more likely to happen, since
the soft timer may be frozen frequently in order to restore the timer
state to the hardware guest timer. This happens after 5-10 hours of
guest soak testing, resulting in an overflow in guest kernel timekeeping
calculations, hanging the guest. A more focussed test case to
intentionally hit the race (with the help of a new hypcall to cause the
timer state to migrated between hardware & software) hits the condition
fairly reliably within around 30 seconds.

Instead of relying purely on the inexact hrtimer expiry to determine
whether an IRQ should be generated, read the guest CP0_Compare and
directly check whether the freeze time is before or after it. Only if
CP0_Count is on or after CP0_Compare do we check the hrtimer expiry to
determine whether the last IRQ has already been generated (which will
have pushed back the expiry by one timer period).

Fixes: e30492bb

 ("MIPS: KVM: Rewrite count/compare timer emulation")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org
Cc: <stable@vger.kernel.org> # 3.16.x-
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>