Re: [Qemu-devel] [PATCH] pc: Clean up PIC-to-APIC IRQ path

[Anthony Liguori]

On 08/31/2011 11:59 AM, Blue Swirl wrote:
> On Wed, Aug 31, 2011 at 8:28 AM, Avi Kivity<address@hidden>  wrote:
> > On 08/30/2011 10:19 PM, Blue Swirl wrote:
> > > >   We need some kind of two phase restore. In the first phase all state is
> > > >   restored; since some of that state drivers outputs that are input to
> > > > other
> > > >   devices, they may experience an edge, and we need to supress that.  In
> > > > the
> > > >   second phase edge detection is unsupressed and the device goes live.
> > >
> > > No. Devices may not perform any externally visible activities (like
> > > toggle a qemu_irq) during or after load because 1) qemu_irq is
> > > stateless and 2) since the receiving end is also freshly loaded, both
> > > states are already in synch without any calls or toggling.
> >
> > That makes it impossible to migrate level-triggered irq lines.  Or at least,
> > the receiver has to remember the state, instead of (or in addition to) the
> > sender.
>
> Both ends probably need to remember the state. That should work
> without any multiphase restores and transient suppressors.
>
> It might be also possible to introduce stateful signal lines which
> save and restore their state, then the receiving end could check what
> is the current level. However, if you consider that the devices may be
> restored in random order, if the IRQ line device happens to be
> restored later, the receiver would still get wrong information. Adding
> priorities could solve this, but I think stateless IRQs are the only
> sane way.

We shouldn't really use the term IRQ as it's confusing.  I like the term 
"pin" better because that describes what we're really talking about.

qemu_irq is designed oddly today because is represents something that is 
intrinsically state (whether a pin is high or low) with an edge 
notification with the assumption that the state is held somewhere else 
(which is usually true).

Modelling stateful pins is useful though for doing something like 
introspecting pin levels, supporting live migration, etc.

The way this works in QOM right now is that the Pin object has a level 
state that can be queried but it also has the ability to register for 
notifications on level change.

The edge change signal isn't registered until realize.  This means that 
you can connect all of the device models, restore all of the pin states, 
and then realize the device model all at once.  At the point of realize, 
all of the devices have the right pin levels so each device can add 
their edge change signals and read the incoming pins and respond 
accordingly.

Regards,

Anthony Liguori

>