Re: [PATCH 0/8] use jump labels to streamline common APIC configuration

[Gleb Natapov <gleb@redhat.com>]

On Sun, Aug 05, 2012 at 05:00:37PM +0300, Avi Kivity wrote:
> On 08/05/2012 04:48 PM, Gleb Natapov wrote:
>  >>
> >> >> During guest boot up, some of these jump keys will change, no?  Does
> >> >> this mean a stop_machine() or equivalent?  I'm worried about real-time
> >> >> response or one guest being affected by another.
> >> >> 
> >> > Yes, SW enable bit changes during boot. The jump label triggerable by a
> >> > guest are rate limited though. So stop machine will not happen more then
> >> > once per second even with malicious guests.
> >> 
> >> I'm not talking about a malicious guest, just a guest that is booting up
> >> normally but kills real-time response for another guest (or just induces
> >> a large hiccup in a non-real-time guest, but we don't guarantee anything
> >> for those).
> >> 
> >> We don't support real-time guests now, but Jan has plans.
> >> 
> > For such setup jump labels have to be compiled out from the kernel
> > completely. Anything that calls stop_machine does not play well with
> > real time.
> > 
> > Guest can cause stop machine on boot today already by detecting PMU and
> > configuring NMI watchdog.
> 
> The host can prevent this by leaving disabling the guest pmu.  But
> disabling jump labels for real-time kernels may be acceptable too.  We
> can probably to it at run time by forcing the slow path at all times.
Yes, it is possible to add module option that will force slow path if
needed.

--
			Gleb.