[Qemu-devel] Re: Guest bridge setup variations

[Alexander Graf]

On 10.12.2009, at 15:18, Arnd Bergmann wrote:

> On Thursday 10 December 2009, Fischer, Anna wrote:
>>> 
>>> 3. Doing the bridging in the NIC using macvlan in passthrough
>>> mode. This lowers the CPU utilization further compared to 2,
>>> at the expense of limiting throughput by the performance of
>>> the PCIe interconnect to the adapter. Whether or not this
>>> is a win is workload dependent. Access controls now happen
>>> in the NIC. Currently, this is not supported yet, due to lack of
>>> device drivers, but it will be an important scenario in the future
>>> according to some people.
>> 
>> Can you differentiate this option from typical PCI pass-through mode?
>> It is not clear to me where macvlan sits in a setup where the NIC does
>> bridging.
> 
> In this setup (hypothetical so far, the code doesn't exist yet), we use
> the configuration logic of macvlan, but not the forwarding. This also
> doesn't do PCI pass-through but instead gives all the logical interfaces
> to the host, using only the bridging and traffic separation capabilities
> of the NIC, but not the PCI-separation.
> 
> Intel calls this mode VMDq, as opposed to SR-IOV, which implies
> the assignment of the adapter to a guest.
> 
> It was confusing of me to call it passthrough above, sorry for that.
> 
>> Typically, in a PCI pass-through configuration, all configuration goes
>> through the physical function device driver (and all data goes directly
>> to the NIC). Are you suggesting to use macvlan as a common
>> configuration layer that then configures the underlying NIC?
>> I could see some benefit in such a model, though I am not certain I
>> understand you correctly.
> 
> This is something I also have been thinking about, but it is not what
> I was referring to above. I think it would be good to keep the three
> cases (macvlan, VMDq, SR-IOV) as similar as possible from the user
> perspective, so using macvlan as an infrastructure for all of them
> sounds reasonable to me.

Oh, so you'd basically do -net vt-d,if=eth0 and the rest would automatically 
work? That's a pretty slick idea!

Alex