Re: how to callback from hypervisor to guest os?

[Jeremy Fitzhardinge <jeremy@goop.org>]

weiming wrote:
> Do you mean making a hypercall after guest domain finishes 
> processing?  If so, in xen, after event_send(), will xxx() be executed 
> immediately (non-blocking)?
>
> Xen:                                              Guest:
>
> _interrupt_handler()                                     
> {
>    ...
>    ...
>    event_send(guest_dom)                event_virq_handler()
> post: xxx()                                  {
>                                                               do_process()
>                                                               
> hypercall_xxx()?
>
>                                                      }
>    ...
> }

You can get Xen to do a callback into the guest.  You can either define 
this as an event callback (probably a virq like the timer or debug 
interrupts), or a specific callback like syscall, event delivery, 
failsafe exceptions etc.  That schedules the guest vcpu running at a 
particular address in kernel context; it can do whatever processing you 
want, then do a hypercall to pass the results back into the hypervisor.

It's a close as you're going to get to a syncronous "call into guest" 
mechanism.  On the hypervisor side you're going to have to deal with it 
as an async operation with split "call into guest" and "get results from 
guest" phases.  You also have to deal with the guest calling the 
hypercall 0-N times - with no correlation to your callbacks, and with 
arbitrary arguments (ie, can't trust the guest's data).

    J