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Re: [Xen-devel] Clock Interrupt not occurring. hypervisor_callback not being called. (more info)

I can't explain why the atomic instructions wouldn't be sufficient.
I just thought it was worth a try to be explicit.
You should be able to create them with 'mb();'


Ivan Kelly wrote:

how would one create a memory barrier? I think the xchg asm
instruction should be a natural barrier in any case.

On 3/1/06, Dave Winchell <dwinchell@xxxxxxxxxxxxxxx> wrote:

How about a memory barrier right before the call to evtchn_notify
in evtchn_set_pending and one before l1 =
x86_atomic_xchg(&s->evtchn_pending_sel, 0)
in do_hypervisor_callback?


Ivan Kelly wrote:

Some more info on this problem.
The problem seems to be that evtchn_pending[0] is not being updated
fast enough after the callback is made. Therefore the callback is
running and seeing nothing as pending and discarding anything that
If i put in something that delays before the start of the inner loop
somewhere, it will work fine. It doesn't have to be a printf, can be a
for(i=0;i<100000;i++) or something similar.
evtchn_pending is set as volatile. Im not sure if this is actually
helping at all though, since I have to use the ACK compiler which is
the only thing that minix will compile with.

Does anyone have any idea on how i can get evtchn_pending to update on time?

do_hypervisor_callback(struct stackframe_s *regs)
      unsigned long  l1, l2;
      unsigned int   l1i, l2i, port;
      int            irq;
      shared_info_t *s = hypervisor_shared_info;

      s->vcpu_data[0].evtchn_upcall_pending = 0;

      l1 = x86_atomic_xchg(&s->evtchn_pending_sel, 0);

      while ( l1 != 0 )
/*              kprintf("l1: %x\n");*/
              l1i = x86_scan_forward(l1);
              l1 &= ~(1 << l1i);

              l2 = s->evtchn_pending[l1i] & ~s->evtchn_mask[l1i];
              while ( l2 != 0 )
                      kprintf("l2: %x\n", l2);
                      l2i = x86_scan_forward(l2);
                      l2 &= ~(1 << l2i);

                      port = (l1i << 5) + l2i;

                      if ( (irq = event_to_action[port]) != -1 ) {
                              do_event(irq, regs);

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