Re: [PATCH 14/22] i8259.c: properly cascade secondary i8259 from primary i8259

[Peter Maydell <peter.maydell@xxxxxxxxxx>]

On Wed, 14 Oct 2026 at 12:20, Mark Cave-Ayland
<mark.caveayland@xxxxxxxxxxx> wrote:
>
> On 13/10/2026 17:18, Peter Maydell wrote:
>
> > On Wed, 16 Sept 2026 at 11:49, Mark Cave-Ayland
> > <mark.caveayland@xxxxxxxxxxx> wrote:
> >>
> >> Currently the cascaded output from the secondary i8259 is returned 
> >> directly in
> >> the pass_irqs[] array, instead of being connected to the relevant input of 
> >> the
> >> primary i8259.
> >>
> >> Signed-off-by: Mark Cave-Ayland <mark.caveayland@xxxxxxxxxxx>
> >> ---
> >>   hw/intc/i8259.c | 3 ++-
> >>   1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/hw/intc/i8259.c b/hw/intc/i8259.c
> >> index fcc94e12f8..964ceb04b2 100644
> >> --- a/hw/intc/i8259.c
> >> +++ b/hw/intc/i8259.c
> >> @@ -501,7 +501,8 @@ static void i8259_pic_realize(DeviceState *dev, Error 
> >> **errp)
> >>       }
> >>
> >>       /* Wire up secondary cascade */
> >> -    qdev_connect_gpio_out(sec_dev, 0, s->pass_irqs[2]);
> >> +    qdev_connect_gpio_out(sec_dev, 0,
> >> +                          qdev_get_gpio_in(pri_dev, 2));
> >
> > We just added this line in the previous patch, so we might as well
> > squash this change into the previous one I think.
> The reason for having this in a separate commit is because it is
> technically a behaviour change. Before this commit, an interrupt from
> the secondary PIC would *always* raise the passed interrupt, whereas on
> real hardware it is possible to disable interrupts from the secondary
> PIC by masking the relevant input on the primary PIC [1].

OK, if it's a behaviour change that's fine. The commit message
is written as if it's just a refactoring, though -- it would
be helpful to spell out what the behaviour change there and
that it's fixing a corner case bug.

-- PMM