Re: [Qemu-devel] Hot-pluggable device without ->unrealize() is highly suspect

[Peter Maydell]

On Fri, 30 Nov 2018 at 07:40, Markus Armbruster <address@hidden> wrote:
> Peter Maydell <address@hidden> writes:
> > Add an assert somewhere and catch it with the usual
> > "instantiate everything" qtest?

> The troublemaker is (3), where we may end up with an overridden
> realize-like method and an non-matching, un-overridden unrealize-like
> method.  Got any smart ideas?

Mmm, I see the difficulty. I suppose in theory we could have
a custom linter like clang-tidy and check that anything that
sets a realize pointer also sets an unrealize, but that's
pie-in-the-sky territory at the moment.

Asserting that there is an unrealize would catch at least
some cases of "forgot to handle hotplug", though, right?

> > More generally, what is causing dc390 to be hotpluggable?
> > I can't see anything obvious in the class init.
>
> It's a PCI device.  These are all hot-pluggable by default.

Maybe we should change that? Most people writing a
PCI device are probably not thinking about hotplug.
Though in the dc390 case it would probably not help to have
to set a dc->hotpluggable flag, because it would inherit that
from am53c974 too.

I wonder if the dc390 could be structured in a way that
it doesn't subclass a complete non-abstract pci device like
that...

> >                                                 If we
> > have APIs where the default is "you get this weird thing
> > you weren't thinking about but it's broken because
> > you weren't thinking about it" then we will have a whole
> > class of bugs that we then need to squash device by device[*].
> > I think it would be better for devices to have to explicitly
> > opt in to implementing things like hotplug -- that way the
> > failure mode is just "this isn't hotpluggable", we can
> > report that helpfully to the user, and if anybody cares
> > they can add the support.
>
> I tend to agree, although for PCI devices, not being hot-pluggable is
> kind of weird, except for the ones that only occur soldered onto
> mainboards.

Real hardware PCI devices are not hot-pluggable by default
as far as I'm aware; PCIe may be hot-pluggable but I think
need to be designed to support it.
This is a random non-hotplug-safe PCIe card:
https://i.stack.imgur.com/tXug5.jpg
PRSNT2# (pin 17 side B) is connected to Ground on pin 18
(these are the rightmost two connections visible); for
hotplug it must be connected to PRSNT1# (which is pin 1 side A),
AIUI.

But the hardware situation is kind of a tangent from how
we try to design our APIs.

thanks
-- PMM