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

[Anthony Liguori <aliguori@us.ibm.com>]

Anthony Liguori wrote:
> Avi Kivity wrote:
>> True, but is it actually useful?
>>
>> Short-circuiting two guest devices probably isn't.  Adding host 
>> devices (like recording packets) might be.
>
> But you can do this by just inserting yourself between the 
> frontend/backend.  You can do this dynamically as long as you don't 
> require feature negotiation.  Dynamic negotiation is prohibitively 
> difficult and one of the primary reasons we haven't been able to merge 
> vnet support for tap yet.

Paul:  in KVM, tap devices support optional GSO/checksum offload 
support.  This is enabled for the tap device depending on whether you're 
using virtio-net and your host kernel supports it.

In KVM, having > 1 guest device on a vlan is broken when a tap device in 
on a vlan.  This is by design because the dynamic feature negotiation 
isn't always possible.  The other option would be to disallow additional 
VLAN that can't support the same features or do full GSO emulation 
within QEMU.

Neither of these are very good options.  No one has complained yet to 
the best of my knowledge that you can't have more than one guest device 
on a vlan in KVM.  This is simply because no one ever does it :-)

-- 
Regards,

Anthony Liguori