Re: [Qemu-devel] [PATCH 09/16] Enable message delivery via IRQs

[Paul Brook <paul@codesourcery.com>]

> On Sat, Jun 12, 2010 at 12:21 PM, Paul Brook <paul@codesourcery.com> wrote:
> >> This patch allows to optionally attach a message to an IRQ event. The
> >> message can contain a payload reference and a callback that the IRQ
> >> handler may invoke to report the delivery result. The former can be used
> >> to model message signaling interrupts, the latter to cleanly implement
> >> IRQ de-coalescing logics.
> > 
> > I don't like this. qemu_irq is a level triggered interface. Redundant
> > calls to qemu_set_irq should (in principle) be a no-op.  If you want
> > message passing then IMO you should be using something else.
> 
> Keeping the optional message and qemu_irq together means that we can
> reuse the existing IRQ subsystem. I'd guess something more separated
> would need duplicate allocation and delivery support and maybe even
> SysBus etc. would need lots of work to support a new class of IRQs.

How do you propose message passing is handled when you have nested multi-layer 
interrupt trees? How long is the message data valid for? Who owns it? How is a 
receiver meant to know for format of the message being delivered, and who it's 
intended for?

IMO message triggered systems are fundamentally different to level states. 
qemu_irq represents a level state, and I'd really like for it to stay that 
way.  

If we need/want a generic message passing interface, then that's a different 
problem, and needs to be done in such a way that the devices always agree on 
the type of message being passed.

TBH I preferred the original system whereby the source can query the state of 
the sink (i.e "are you ignoring this line?").  Note that conceptually this 
should be *querying* state, not responding to an event.

Paul