From: Arnd Bergmann [mailto:arnd@arndb.de]
Sent: Tuesday, September 14, 2010 11:21 PM
To: Shirley Ma
Cc: Avi Kivity; David Miller; mst@redhat.com; Xin, Xiaohui; netdev@vger.kernel.org;
kvm@vger.kernel.org; linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 2/2] macvtap: TX zero copy between guest and host kernel

On Tuesday 14 September 2010, Shirley Ma wrote:

quoted

On Tue, 2010-09-14 at 11:12 +0200, Avi Kivity wrote:

quoted

That's what io_submit() is for.  Then io_getevents() tells you what
"a
while" actually was.

This macvtap zero copy uses iov buffers from vhost ring, which is
allocated from guest kernel. In host kernel, vhost calls macvtap
sendmsg. macvtap sendmsg calls get_user_pages_fast to pin these buffers'
pages for zero copy.

The patch is relying on how vhost handle these buffers. I need to look
at vhost code (qemu) first for addressing the questions here.

I guess the best solution would be to make macvtap_aio_write return
-EIOCBQUEUED when a packet gets passed down to the adapter, and
call aio_complete when the adapter is done with it.

This would change the regular behavior of macvtap into a model where
every write on the file blocks until the packet has left the machine,
which gives us better flow control, but does slow down the traffic
when we only put one packet at a time into the queue.

It also allows the user to call io_submit instead of write in order
to do an asynchronous submission as Avi was suggesting.

But currently, this patch is communicated with vhost-net, which is almost
in the kernel side. If it uses aio stuff, it should be communicate with user
space Backend. 
 

Arnd