Re: [Qemu-devel] Register uhci_reset() callback.

[Paul Brook <paul@codesourcery.com>]

> >  Devices should not cause IRQ state changes on restore. Commit 3dcd219f
> > is incorrect.
>
> I'm not so sure about this, but I can't think of a restore sequence
> where the IRQ state would need to be changed if the IRQs tied together
> are handled correctly. But surely if the devices states are restored
> in strange order, the state changes could cause problems because the
> device receiving the IRQ may still contain old state.

It's precisely because devices are restored in unpredictable order that they 
should not be communicating with other devices (e.g. by modifying IRQ lines).

Consider a system with a device (DEV) and a level triggered interrupt 
controller (PIC1) chained to an edge triggered interrupt controller (PIC2).

(DEV) ->  (PIC1) -> (PIC2)

Before restore, DEV output is low, PIC1 has the interrupt unmasked (but low), 
PIC2 has no pending interrupts.

We now restore a state where DEV output is high, PIC1 has masked the 
interrupt, and PIC2 has no pending interrupts. Devices are restored in he 
order PIC2, DEV, PIC1.

If devices toggle their interrupts on restore then we get incorrect state 
after the restore:

PIC2 is restored to the desired no-interrupts-pending state.
DEV is restored. This raises the IRQ, which is passed to PIC1. PIC1 still has 
the old interrupt mask, so passes through to PIC2, which detects the edge 
event and marks the interrupt as pending.
PIC1 is restored, updates the new mask and lowers its output. However this 
does not clear the internal PIC2 pending interrupt flag, so machine state will 
be wrong after resume.

Paul