On Fri, 2006-11-17 at 15:29 +0000, Grzegorz Milos wrote:
> > Suppose a callback occurs between the calls of __sti() and
> > init_events(). The function do_event() calls the handler indirectly via
> > the array ev_actions. But ev_actions is initialised in init_events(),
> > so if do_event() is called too early, the function pointer "handler" in
> > ev_actions will still be 0 (default for static storage). We start again
> > at virtual address 0, which is the entry point of Mini-OS, but %esi will
> > not now point to the start_info page. I think this explains why Mini-OS
> > sometimes gets "restarted", and when it does the start_info page seems
> > to be garbage.
>
> This seems to explain the "restart". However, my understanding was that
> an event port has to be explicitly unmasked in order for any event to
> be delivered (i.e. that all event ports are masked by default).
> Unmasking is done by bind_evtchn() or bind_virq() in event.s. All calls
> to these functions happen after init_events() (in init_time(),
> init_console() and init_xenbus()). I guess the best way to verify if
> your scenario is correct is to put 'printk' in do_events(). Could you
> try this Melvin?
Very good question. I have just tried calling a modified version of
init_events() to set up the default_handler before __sti() is called,
but my modified init_events() did not call mask_evtchn().
I find that I occasionally see an event on port 2. Looking at the debug
output from xend (I ran xend with XEND_DEBUG=1 set in the environment),
I see that this is the console event port.
I have just looked at the domain builder code in libxc,
tools/libxc/xc_linux_build.c, function setup_guest(). I see that this
initialises the shared_info page with:
memset(shared_info, 0, PAGE_SIZE);
/* Mask all upcalls... */
for ( i = 0; i < MAX_VIRT_CPUS; i++ )
shared_info->vcpu_info[i].evtchn_upcall_mask = 1;
So, the evtchn_mask array is cleared (unmasked) by the memset() call,
but the per-cpu evtchn_upcall_mask is set to hold off callbacks until we
are properly initialised. Function __sti() clears evtchn_upcall_mask,
and as evtchn_mask array is cleared by default, that explains why the
upcall on port 2 is not masked.
Regards,
Melvin Anderson.
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