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

[Markus Armbruster <armbru@redhat.com>]

"Michael S. Tsirkin" <mst@redhat.com> writes:

> 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.

Gerd, what do you think about changing the contract for property parse
methods to return -EINVAL, -EBUSY and such?