On Thu, 16 Jul 2020 16:01:18 +0200
Markus Armbruster <arm...@redhat.com> wrote:
> David Gibson <da...@gibson.dropbear.id.au> writes:
>
> > On Thu, Jul 09, 2020 at 07:12:47PM +0200, Greg Kurz wrote:
> >> Some recent error handling cleanups unveiled issues with our support of
> >> PCI bridges:
> >>
> >> 1) QEMU aborts when using non-standard PCI bridge types,
> >> unveiled by commit 7ef1553dac "spapr_pci: Drop some dead error handling"
> >>
> >> $ qemu-system-ppc64 -M pseries -device pcie-pci-bridge
> >> Unexpected error in object_property_find() at qom/object.c:1240:
> >> qemu-system-ppc64: -device pcie-pci-bridge: Property '.chassis_nr' not
> >> found
> >> Aborted (core dumped)
> >
> > Oops, I thought we had a check that we actually had a "pci-bridge"
> > device before continuing with the hotplug, but I guess not.
> >
> >> This happens because we assume all PCI bridge types to have a "chassis_nr"
> >> property. This property only exists with the standard PCI bridge type
> >> "pci-bridge" actually. We could possibly revert 7ef1553dac but it seems
> >> much simpler to check the presence of "chassis_nr" earlier.
> >
> > Hrm, right, 7ef1553dac was not really correct since add_drcs() really
> > can fail.
>
> Right. I failed to see that we can run into a bridge without a
> "chassis_nr" here.
>
> >> 2) QEMU abort if same "chassis_nr" value is used several times,
> >> unveiled by commit d2623129a7de "qom: Drop parameter @errp of
> >> object_property_add() & friends"
> >>
> >> $ qemu-system-ppc64 -M pseries -device pci-bridge,chassis_nr=1 \
> >> -device pci-bridge,chassis_nr=1
> >> Unexpected error in object_property_try_add() at qom/object.c:1167:
> >> qemu-system-ppc64: -device pci-bridge,chassis_nr=1: attempt to add
> >> duplicate property '40000100' to object (type 'container')
> >> Aborted (core dumped)
>
> Before d2623129a7de, the error got *ignored* in
> spapr_dr_connector_new():
>
> SpaprDrc *spapr_dr_connector_new(Object *owner, const char *type,
> uint32_t id)
> {
> SpaprDrc *drc = SPAPR_DR_CONNECTOR(object_new(type));
> char *prop_name;
>
> drc->id = id;
> drc->owner = owner;
> prop_name = g_strdup_printf("dr-connector[%"PRIu32"]",
> spapr_drc_index(drc));
> object_property_add_child(owner, prop_name, OBJECT(drc),
> &error_abort);
> object_unref(OBJECT(drc));
> ---> object_property_set_bool(OBJECT(drc), true, "realized", NULL);
> g_free(prop_name);
>
> return drc;
> }
>
> I doubt that's healthy.
>
This isn't. The object_property_set_bool() was later converted to
qdev_realize() (thanks again for the cleanups!) but the problem
remains. Realize can fail and I see now reason we don't do proper
error handling when it comes to the DRCs.
I'll look into fixing that.
> >> This happens because we assume that "chassis_nr" values are unique, but
> >> nobody enforces that and we end up generating duplicate DRC ids. The PCI
> >> code doesn't really care for duplicate "chassis_nr" properties since it
> >> is only used to initialize the "Chassis Number Register" of the bridge,
> >> with no functional impact on QEMU. So, even if passing the same value
> >> several times might look weird, it never broke anything before, so
> >> I guess we don't necessarily want to enforce strict checking in the PCI
> >> code now.
> >
> > Yeah, I guess. I'm pretty sure that the chassis number of bridges is
> > supposed to be system-unique (well, unique within the PCI domain at
> > least, I guess) as part of the hardware spec. So specifying multiple
> > chassis ids the same is a user error, but we need a better failure
> > mode.
> >
> >> Workaround both issues in the PAPR code: check that the bridge has a
> >> unique and non null "chassis_nr" when plugging it into its parent bus.
> >>
> >> Fixes: 05929a6c5dfe ("spapr: Don't use bus number for building DRC ids")
> >
> > Arguably, it's really fixing 7ef1553dac.
>
> I agree 7ef1553dac broke the "use a bridge that doesn't have property
> 'chassis_nr' case.
>
> I suspect the "duplicate chassis_nr" case has always been broken, and
> d2623129a7de merely uncovered it.
>
Yes.
> If we can trigger the abort with hot-plug, then d2623129a7de made things
> materially worse (new way to accidentally kill your guest and maybe lose
> data), and I'd add a Fixes: blaming it.
>
Yes it does.
David,
Maybe consider folding a third Fixes: tag into this patch ?
> >> Reported-by: Thomas Huth <th...@redhat.com>
> >> Signed-off-by: Greg Kurz <gr...@kaod.org>
> >
> > I had a few misgivings about the details of this, but I think I've
> > convinced myself they're fine. There's a couple of things I'd like to
> > polish, but I'll do that as a follow up.
>
