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[libvirt] Re: Supporting vhost-net and macvtap in libvirt for QEMU



Daniel P. Berrange wrote:
The more interesting invocation of vhost-net though is one where the vhost-net device backs directly to a physical network card. In this mode, vhost should get considerably better performance than the current implementation. I don't know the syntax yet, but I think it's reasonable to assume that it will look something like -net tap,dev=eth0. The effect will be that eth0 is dedicated to the guest.

Ok, so in this model you have to create a dedicated ethXX device for
every guest, no sharing ?

Yup. You may be sharing a physical network device via SR-IOV, but from libvirt's perspective, we're dedicating a physical device to a guest virtual nic.

I think there are a few ways libvirt could support vhost-net in this second mode. The simplest would be to introduce a new tag similar to <source network='br0'>. In fact, if you probed the device type for the network parameter, you could probably do something like <source network='eth0'> and have it Just Work.

Another model would be to have libvirt see an SR-IOV adapter as a network pool whereas it handled all of the VF management. Considering how inflexible SR-IOV is today, I'm not sure whether this is the best model.

Agreed, given the hardware limitations I don't see that it is worth the
bother.
This new mode is not really what we'd call 'bridging' in libvirt network
XML format, so I think we'll want to define a new type of network config
for it in libvirt. Perhaps
  <network type='physical'>
    <source dev='eth0'/>
  </network>

Or type='passthru'

That certainly simplifies the problem.

I don't know whether SR-IOV requires additional setup though wrt programming the VF's mac address. It may make sense for libvirt to at least do that.

--
Regards,

Anthony Liguori


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