Re: [Xen-devel] Real-time support

["Geoffrey Lefebvre" <geoffrey@xxxxxxxxx>]

> The pin/unpin operations are certainly by far the longest running
> operations in Xen, and it's been on the to-do list to make them
> preemptable for a long time. This should be very simple as we can exit
> the hypervisor leaving the EIP on the hypercall, and next time the guest
> calls in we'll pick up where we left off.
Hi,

I have thought about real-time support in the past and depending on
the type of real-time guest you want to run, I believe the problem
might be more complicated than it first seems but could be wrong. Feel
free to enlighten me if  my comments below don't make sense :)

I think if all you want to do is preempt one guest to be able to run a
different guest then the solution as stated above will work.  Things
get more complicated if you want to be able to deliver an event to a
guest while that same guest is trapped in the hypervisor.  You
potentially want this feature to avoid a scenario such as a low
priority thread that invoked a long hypercall delaying a high priority
thread waiting for an event (such as a timer). The problem is that
once you allow this upcall into the guest, there is no guarantee that
the next hypercall will be the re-execution of the hypercall you
preempted.

If the real-time guest you are running is quite simple (a la mini-os)
then you can avoid this kind of scenario but the problem gets harder
to avoid (i think) if you are running something like Linux + preempt
RT as a guest.

cheers,

geoffrey

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