[Qemu-devel] Re: Guest bridge setup variations

[Arnd Bergmann]

On Wednesday 16 December 2009, Leonid Grossman 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. 
> 
> This is certainly true today for pci-e 1.1 and 2.0 devices, but 
> as NICs move to pci-e 3.0 (while remaining almost exclusively dual port
> 10GbE for a long while), 
> EVB internal bandwidth will significantly exceed external bandwidth.
> So, #3 can become a win for most inter-guest workloads.

Right, it's also hardware dependent, but it usually comes down
to whether it's cheaper to spend CPU cycles or to spend IO bandwidth.

I would be surprised if all future machines with PCIe 3.0 suddenly have
a huge surplus of bandwidth but no CPU to keep up with that.

> > > 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.
> 
> Actually, x3100 10GbE drivers support this today via sysfs interface to
> the host driver 
> that can choose to control VEB tables (and therefore MAC addresses, vlan
> memberships, etc. for all passthru interfaces behind the VEB).

Ok, I didn't know about that.

> OF course a more generic vendor-independent interface will be important
> in the future.

Right. I hope we can come up with something soon. I'll have a look at
what your driver does and see if that can be abstracted in some way.
I expect that if we can find an interface between the kernel and device
driver for two or three NIC implementations that it will be good enough
to adapt to everyone else as well.

        Arnd