Re: [PATCH 09/11] qdev: Avoid QemuOpts in QMP device_add

On 10/5/21 16:37, Kevin Wolf wrote: > Am 27.09.2021 um 13:39 hat Kevin Wolf geschrieben: > > Am 27.09.2021 um 13:06 hat Damien Hedde geschrieben: > > > On 9/24/21 11:04, Kevin Wolf wrote: > > > > Directly call qdev_device_add_from_qdict() for QMP device_add instead of > > > > first going through QemuOpts and converting back to QDict. > > > > > > > > Note that this changes the behaviour of device_add, though in ways that > > > > should be considered bug fixes: > > > > > > > > QemuOpts ignores differences between data types, so you could > > > > successfully pass a string "123" for an integer property, or a string > > > > "on" for a boolean property (and vice versa). After this change, the > > > > correct data type for the property must be used in the JSON input. > > > > > > > > qemu_opts_from_qdict() also silently ignores any options whose value is > > > > a QDict, QList or QNull. > > > > > > > > To illustrate, the following QMP command was accepted before and is now > > > > rejected for both reasons: > > > > > > > > { "execute": "device_add", > > > > "arguments": { "driver": "scsi-cd", > > > > "drive": { "completely": "invalid" }, > > > > "physical_block_size": "4096" } } > > > > > > > > Signed-off-by: Kevin Wolf <kwolf(a)redhat.com&gt; > > > > --- > > > > softmmu/qdev-monitor.c | 18 +++++++++++------- > > > > 1 file changed, 11 insertions(+), 7 deletions(-) > > > > > > > > diff --git a/softmmu/qdev-monitor.c b/softmmu/qdev-monitor.c > > > > index c09b7430eb..8622ccade6 100644 > > > > --- a/softmmu/qdev-monitor.c > > > > +++ b/softmmu/qdev-monitor.c > > > > @@ -812,7 +812,8 @@ void hmp_info_qdm(Monitor *mon, const QDict *qdict) > > > > qdev_print_devinfos(true); > > > > } > > > > -void qmp_device_add(QDict *qdict, QObject **ret_data, Error **errp) > > > > +static void monitor_device_add(QDict *qdict, QObject **ret_data, > > > > + bool from_json, Error **errp) > > > > { > > > > QemuOpts *opts; > > > > DeviceState *dev; > > > > @@ -825,7 +826,9 @@ void qmp_device_add(QDict *qdict, QObject **ret_data, Error **errp) > > > > qemu_opts_del(opts); > > > > return; > > > > } > > > > - dev = qdev_device_add(opts, errp); > > > > + qemu_opts_del(opts); > > > > + > > > > + dev = qdev_device_add_from_qdict(qdict, from_json, errp); > > > > > > Hi Kevin, > > > > > > I'm wandering if deleting the opts (which remove it from the "device" opts > > > list) is really a no-op ? > > > > It's not exactly a no-op. Previously, the QemuOpts would only be freed > > when the device is destroying, now we delete it immediately after > > creating the device. This could matter in some cases. > > > > The one case I was aware of is that QemuOpts used to be responsible for > > checking for duplicate IDs. Obviously, it can't do this job any more > > when we call qemu_opts_del() right after creating the device. This is > > the reason for patch 6. > > > > > The opts list is, eg, traversed in hw/net/virtio-net.c in the function > > > failover_find_primary_device_id() which may be called during the > > > virtio_net_set_features() (a TYPE_VIRTIO_NET method). > > > I do not have the knowledge to tell when this method is called. But If this > > > is after we create the devices. Then the list will be empty at this point > > > now. > > > > > > It seems, there are 2 other calling sites of > > > "qemu_opts_foreach(qemu_find_opts("device"), [...]" in net/vhost-user.c and > > > net/vhost-vdpa.c > > > > Yes, you are right. These callers probably need to be changed. Going > > through the command line options rather than looking at the actual > > device objects that exist doesn't feel entirely clean anyway. > > So I tried to have a look at the virtio-net case, and ended up very > confused. > > Obviously looking at command line options (even of a differrent device) > from within a device is very unclean. With a non-broken, i.e. type safe, > device-add (as well as with the JSON CLI option introduced by this > series), we can't have a QemuOpts any more that is by definition unsafe. > So this code needs a replacement. > > My naive idea was that we just need to look at runtime state instead. > Don't search the options for a device with a matching 'failover_pair_id' > (which, by the way, would fail as soon as any other device introduces a > property with the same name), but search for actual PCIDevices in qdev > that have pci_dev->failover_pair_id set accordingly. > > However, the logic in failover_add_primary() suggests that we can have a > state where QemuOpts for a device exist, but the device doesn't, and > then it hotplugs the device from the command line options. How would we > ever get into such an inconsistent state where QemuOpts contains a > device that doesn't exist? Normally devices get their QemuOpts when they > are created and device_finalize() deletes the QemuOpts again. > Just read the following from docs/system/virtio-net-failover.rst > Usage > ----- > > The primary device can be hotplugged or be part of the startup > configuration > > -device virtio-net-pci,netdev=hostnet1,id=net1, > mac=52:54:00:6f:55:cc,bus=root2,failover=on > > With the parameter failover=on the VIRTIO_NET_F_STANDBY feature > will be enabled. > > -device vfio-pci,host=5e:00.2,id=hostdev0,bus=root1, > failover_pair_id=net1 > > failover_pair_id references the id of the virtio-net standby device. > This is only for pairing the devices within QEMU. The guest kernel > module net_failover will match devices with identical MAC addresses. > > Hotplug > ------- > > Both primary and standby device can be hotplugged via the QEMU > monitor. Note that if the virtio-net device is plugged first a > warning will be issued that it couldn't find the primary device. So maybe this whole primary device lookup can happen during the -device CLI option creation loop. And we can indeed have un-created devices still in the list ?

> Any suggestions how to get rid of the QemuOpts abuse in the failover > code? > > If this is a device that we previously managed to rip out without > deleting its QemuOpts, can we store its dev->opts (which is a type safe > QDict after this series) somewhere locally instead of looking at global > state? Preferably I would even like to get rid of dev->opts because we > really should look at live state rather than command line options after > device creation, but I guess one step at a time. > > (Actually, I'm half tempted to just break it because no test cases seem > to exist, so apparently nobody is really interested in it.) > > Kevin >