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

[Edgar E. Iglesias]

On Sat, Sep 03, 2011 at 02:53:31PM -0500, Anthony Liguori wrote:
> 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).

I don't agree. That's not what qemu_irq represents.
It represents a wire, a mechanism to drive changes through logic paths
between state. It is intrinsically stateless.

It may be the case that it is missused in some places, or that it isn't
always the best thing to use to represent what ever you need to represent,
so that you want to complement with other mechanisms.
But universally replacing it with a stateful alternative seems wrong to me.

Cheers