Re: [Xen-devel] fooey. no interrupts.

[Christian Limpach <Christian.Limpach@xxxxxxxxxxxx>]

On Tue, Aug 10, 2004 at 02:35:08PM +0100, Keir Fraser wrote:
> > On Tue, 10 Aug 2004, Keir Fraser wrote:
> > 
> > > 
> > > > >  4. The original value of vcpu_data[0].evtchn_upcall_pending must be 
> > > > > zero.
> > > > >  5. The value of vcpu_data[0].evtchn_upcall_mask must be zero.
> > > > 
> > > > OK, this I don't totally see. From the code I posted before, it seems 
> > > > to 
> > > > me only the first three conditions matter. 
> > > 
> > > The first three conditions cause us to decide whether or not to
> > > schedule the target domain, sending a cross-cpu interrupt if
> > > necessary. The final two are checks just before calling back to the
> > > guest OS, just to check whether it is in a position to receive async
> > > callbacks. 
> > 
> > Keir, I don't see that in the code and Christian sent a note that left me 
> > thinking it does not work that way. 
> > 
> > as Christian said, (4) doesn't do anything conditional, it does this:
> >         /* The VCPU pending flag must be set /after/ update to 
> > evtchn-pend. */
> >         s->vcpu_data[0].evtchn_upcall_pending = 1;
> >         guest_async_callback(d);
> > 
> > which looks pretty unconditional to me. Is there something else I'm 
> > missing?
> 
> No, I'd forgotten how the code worked -- Christian is correct.
> evtchn_upcall_pending is set unconditionally on the CPU that is
> transmitting the event. evtchn_upcall_mask is only checked immediately
> before return to your guest OS to determine whether or not to create
> an async callback frame.

For completeness sake:  evtchn_upcall_pending is also checked
immediately before return to your guest OS to determine whether
or not to create an async callback frame.  Unlike evtchn_upcall_mask
evtchn_upcall_pending needs to be set while evtchn_upcall_mask needs
to be clear.
See /*test_guest_events:*/ in xen/arch/x86/x86_32/entry.S

    christian



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