netdev - Re: [RFC PATCH 2/2] macvtap: TX zero copy between guest and host kernel

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Message-Id: <201009141721.13202.arnd@arndb.de>
Date:	Tue, 14 Sep 2010 17:21:13 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Shirley Ma <mashirle@...ibm.com>
Cc:	Avi Kivity <avi@...hat.com>, David Miller <davem@...emloft.net>,
	mst@...hat.com, xiaohui.xin@...el.com, netdev@...r.kernel.org,
	kvm@...r.kernel.org, linux-kernel@...r.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:
> On Tue, 2010-09-14 at 11:12 +0200, Avi Kivity wrote:
>
> > 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.

	Arnd
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