Re: [Qemu-devel] [opnfv-tech-discuss] rfc: vhost user enhancements for vm2vm communication

[Zhang, Yang Z]

Michael S. Tsirkin wrote on 2015-09-13:
> On Fri, Sep 11, 2015 at 05:39:07PM +0200, Claudio Fontana wrote:
>> On 09.09.2015 09:06, Michael S. Tsirkin wrote:
>> 
>> There are many consequences to this, offset within BAR alone is not
>> enough, there are multiple things at the virtio level that need sorting
>> out. Also we need to consider virtio-mmio etc.
>> 
>>> This would allow VM2VM communication if there are only 2 VMs, but
>>> if data needs to be sent to multiple VMs, you must copy it.
>> 
>> Not necessarily, however getting it to work (sharing the backend window
>> and arbitrating the multicast) is really hard.
>> 
>>> 
>>> Additionally, it's a single-purpose feature: you can use it from a
>>> userspace PMD but linux will never use it.
>>> 
>>> 
>>> My proposal is a superset: don't require that BAR memory is used,
>>> use IOMMU translation tables.
>>> This way, data can be sent to multiple VMs by sharing the same
>>> memory with them all.
>> 
>> Can you describe in detail how your proposal deals with the
>> arbitration
> necessary for multicast handling?
> 
> Basically it falls out naturally. Consider linux guest as an example,
> and assume dynamic mappings for simplicity.
> 
> Multicast is done by a bridge on the guest side. That code clones the
> skb (reference-counting page fragments) and passes it to multiple ports.
> Each of these will program the IOMMU to allow read access to the
> fragments to the relevant device.

How to work with vswitch in host side like OVS? Since the flow table is inside 
host, but guest cannot see it.

Best regards,
Yang