Re: -nic model=rocker causes qemu to abort

[Michael Tokarev <mjt@tls.msk.ru>]

25.05.2020 21:45, Thomas Huth wrote:
> On 29/04/2020 18.43, Michael Tokarev wrote:
>> Hi!
>>
>> Just a fun case of (invalid) usage of qemu-system command line.
>> Someone tried -nic model=rocker, and qemu does this:
>>
>>  Unexpected error in object_property_find() at /build/qemu/git/qom/object.c:1029:
>>  qemu-system-x86_64: Property '.mac' not found
>>  Aborted
>>
>> This happens after this commit:
>>
>> commit 52310c3fa7dc854dd2376ae8a518141abcdb78f3
>> Author: Paolo Bonzini <pbonzini@redhat.com>
>> Date:   Fri Mar 2 10:30:50 2018 +0100
>>
>>     net: allow using any PCI NICs in -net or -nic
>>
>> Previously rocker rightly wasn't usable as a nic model,
>> and after this commit it is now possible.
>>
>> While I agree this is invalid usage, perhaps qemu should not
>> abort like this?  Maybe it should check the required property
>> too, before allowing this device to be a nic model?
> 
> QEMU theoretically should never abort() - abort() means there is
> something wrong in the code ... so we should definitely fix this by
> other means. Some questions:
> 
> 1) How is that rocker device normally used? Similar to a normal network
> card? Or completely different?

Nope, this is not a network card, it is a network SWITCH, used as a
regular PCI device, not network-specific. Hence the problem at hand, -
before the patch this device weren't "nic-able", so to say, because
while it is network-related PCI device, it is not a NIC.

I guess while building a list of "nic-able" cards, we should skip some
"nic-alike" devices and use some more specific criteria. Not everything
which is network-related is a regular NIC.

Maybe in time some other devices will appear in qemu with similar effect
which will require another condition.

Or adding a new field into the pci structure to indicate this device
can be a NIC will solve it once for all. Or just reverting the patch :) -
after all it isn't that bad to have a list of NIC-able devices outside
of the device definitions, - we don't have tons of devices anyway where
maintaining such a list outside of device model is a problem.

Thanks,

/mjt

> 2) In case it is similar to a normal network card, would it make sense
> to allow it with -nic or -net, too? Or should we simply disallow it
> here? I think we could either use a list of devices that should never be
> allowed here, or we check for the availability of that "mac" property...
> 
>  Thomas
>