Re: [Qemu-devel] Re: [PATCH 9/9] Introduce VLANClientState::cleanup()

[Avi Kivity <avi@redhat.com>]

Paul Brook wrote:
>>> I'm still not understanding. Ethernet devices are fundamentally based
>>> around a bus architecture. "flip the TX/RX queues" only makes sense if
>>> you're talking about a point-point connection. For ethernet devices I see
>>> no reason to distinguish between "host" devices (slirp, vde, tap) and
>>> "guest" devices. They may be created for different reasons, but they're
>>> all doing fundamentally the same thing.
>>>       
>> In the bus model, there's an implicit copy-to-the-wire operation that
>> results in replication of the packet to everything else on the bus.
>>  From a performance perspective, this is not at all ideal.
>>     
>
> I'm not convinced by this argument. I don't see why a one-many api requires a 
> copy.
>   

To achieve copyless receive (from guest perspective), you need to give 
the receive buffer to the transmitting device (tap).  If there are more 
than one transmitting devices, you have a conflict.

Sure, you can short-ciruit the case where you have a pair of devices, 
but then you need to revoke posted receive descriptors.

-- 
error compiling committee.c: too many arguments to function