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

[Blue Swirl <blauwirbel@gmail.com>]

On 6/16/09, Paul Brook <paul@codesourcery.com> wrote:
> > >  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.

Yes, this is the "bad" restore scenario that I had in mind. I still
have a nagging feeling that there is a reverse scenario, where in
order to reach good state, you would have to call the IRQ function.
Anyway, 3dcd219f may not be correct in that case either.