Re: [PATCHv5 3/3] vhost_net: a kernel-level virtio server

From: Arnd Bergmann
Date: Mon Aug 31 2009 - 11:25:43 EST

On Monday 31 August 2009, Xin, Xiaohui wrote:
> Hi, Michael
> That's a great job. We are now working on support VMDq on KVM, and since the VMDq hardware presents L2 sorting
> based on MAC addresses and VLAN tags, our target is to implement a zero copy solution using VMDq.

I'm also interested in helping there, please include me in the discussions.

> We stared
> from the virtio-net architecture. What we want to proposal is to use AIO combined with direct I/O:
> 1) Modify virtio-net Backend service in Qemu to submit aio requests composed from virtqueue.

right, that sounds useful.

> 2) Modify TUN/TAP device to support aio operations and the user space buffer directly mapping into the host kernel.
> 3) Let a TUN/TAP device binds to single rx/tx queue from the NIC.

I don't think we should do that with the tun/tap driver. By design, tun/tap is a way to interact with the
networking stack as if coming from a device. The only way this connects to an external adapter is through
a bridge or through IP routing, which means that it does not correspond to a specific NIC.

I have worked on a driver I called 'macvtap' in lack of a better name, to add a new tap frontend to
the 'macvlan' driver. Since macvlan lets you add slaves to a single NIC device, this gives you a direct
connection between one or multiple tap devices to an external NIC, which works a lot better than when
you have a bridge inbetween. There is also work underway to add a bridging capability to macvlan, so
you can communicate directly between guests like you can do with a bridge.

Michael's vhost_net can plug into the same macvlan infrastructure, so the work is complementary.

> 4) Modify the net_dev and skb structure to permit allocated skb to use user space directly mapped payload
> buffer address rather then kernel allocated.


> As zero copy is also your goal, we are interested in what's in your mind, and would like to collaborate with you if possible.
> BTW, we will send our VMDq write-up very soon.

Ok, cool.

Arnd <><
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