[Qemu-devel] Re: Guest bridge setup variations

[Arnd Bergmann]

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.

The difference between VMDq and SR-IOV in that case would be
that the former uses a virtio-net driver in the guest and a hardware
driver in the host, while the latter uses a hardware driver in the guest
only. The data flow on these two would be identical though, while
in the classic macvlan the data forwarding decisions are made in
the host kernel.

        Arnd