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

[Kevin Wolf <kwolf@redhat.com>]

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@redhat.com>
> > > ---
> > >   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.

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