[Qemu-devel] Re: [PATCH] qdev: Catch attempt to attach more than one device to a netdev

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Feb 25, 2010 at 01:07:54PM +0100, Markus Armbruster wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
> 
> > On Thu, Feb 25, 2010 at 11:10:15AM +0100, Markus Armbruster wrote:
> >> Guest device and host netdev are peers, i.e. it's a 1:1 relation.
> >> However, we fail to enforce that:
> >> 
> >>     $ qemu -nodefaults --nographic -netdev user,id=net0 -device e1000,netdev=net0 -device virtio-net-pci,netdev=net0 -monitor stdio
> >>     QEMU 0.12.50 monitor - type 'help' for more information
> >>     (qemu) info network
> >>     Devices not on any VLAN:
> >>       net0: net=10.0.2.0, restricted=n peer=virtio-net-pci.0
> >>       e1000.0: model=e1000,macaddr=52:54:00:12:34:56 peer=net0
> >>       virtio-net-pci.0: model=virtio-net-pci,macaddr=52:54:00:12:34:57 peer=net0
> >> 
> >> It's all downhill from there.
> >> 
> >> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> >> ---
> >>  hw/qdev-properties.c |    3 +++
> >>  net.c                |    1 +
> >>  2 files changed, 4 insertions(+), 0 deletions(-)
> >> 
> >> diff --git a/hw/qdev-properties.c b/hw/qdev-properties.c
> >> index 277ff9e..89efd91 100644
> >> --- a/hw/qdev-properties.c
> >> +++ b/hw/qdev-properties.c
> >> @@ -341,6 +341,9 @@ static int parse_netdev(DeviceState *dev, Property *prop, const char *str)
> >>      *ptr = qemu_find_netdev(str);
> >>      if (*ptr == NULL)
> >>          return -1;
> >> +    if ((*ptr)->peer) {
> >> +        return -1;
> >> +    }
> >
> > Not even -EBUSY?
> > Can this produce a helpful diagnostic message?
> 
> Callers take care of that.  It's how property parse methods work.
> In this case:
> 
>     property "virtio-net-pci.netdev": failed to parse "net0"
>     can't set property "netdev" to "net0" for "virtio-net-pci"

Yes, but does not tell you why. If parse_netdev even just returned a meaningful
error code, we could see:
    can't set property "netdev" to "net0" for "virtio-net-pci":
    device or resource busy.

which is at least a hint that something else uses it.

As it is, it looks like qemu could not parse "net0"
which is not really right.


> >>      return 0;
> >>  }
> >>  
> >> diff --git a/net.c b/net.c
> >> index a1bf49f..e6c96d3 100644
> >> --- a/net.c
> >> +++ b/net.c
> >> @@ -245,6 +245,7 @@ VLANClientState *qemu_new_net_client(NetClientInfo *info,
> >>          QTAILQ_INSERT_TAIL(&vc->vlan->clients, vc, next);
> >>      } else {
> >>          if (peer) {
> >> +            assert(!peer->peer);
> >
> > Do we ever get herE?
> 
> If "get here" means "conditional entered": yes.  For instance, with the
> command line from the commit message, we get here for each of the
> -device, like this:
> 
> #0  qemu_new_net_client (info=0x8b2ce0, vlan=0x0, peer=0xcd83f0, 
>     model=0x5ea826 "e1000", name=0x0) at /work/armbru/qemu/net.c:227
> #1  0x000000000047682f in qemu_new_nic (info=0x8b2ce0, conf=0x7fffece4a270, 
>     model=0x5ea826 "e1000", name=0x0, opaque=0x7fffece4a010)
>     at /work/armbru/qemu/net.c:273
> #2  0x0000000000447c74 in pci_e1000_init (pci_dev=0x7fffece4a010)
>     at /work/armbru/qemu/hw/e1000.c:1125
> #3  0x0000000000422335 in pci_qdev_init (qdev=0x7fffece4a010, base=0x8b2dc0)
>     at /work/armbru/qemu/hw/pci.c:1647
> #4  0x00000000004bbb29 in qdev_init (dev=0x7fffece4a010)
>     at /work/armbru/qemu/hw/qdev.c:265
> 
> If "get here" means "assertion fails": only if somebody breaks something
> elsewhere.  And then assertion will slap his wrist.

I see.

> >>              vc->peer = peer;
> >>              peer->peer = vc;
> >>          }
> >> -- 
> >> 1.6.6
> >> 
> >>