linux-kernel - Re: [PATCH RFC] tun: experimental zero copy tx support

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Message-ID: <20120514191719.GD17086@redhat.com>
Date:	Mon, 14 May 2012 22:17:20 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Stephen Hemminger <shemminger@...tta.com>,
	"David S. Miller" <davem@...emloft.net>,
	Joe Perches <joe@...ches.com>,
	Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, Ian.Campbell@...rix.com,
	kvm@...r.kernel.org
Subject: Re: [PATCH RFC] tun: experimental zero copy tx support

On Mon, May 14, 2012 at 10:14:56PM +0300, Michael S. Tsirkin wrote:
> On Mon, May 14, 2012 at 09:02:26PM +0200, Eric Dumazet wrote:
> > On Mon, 2012-05-14 at 20:04 +0300, Michael S. Tsirkin wrote:
> > > On Mon, May 14, 2012 at 09:54:46AM -0700, Stephen Hemminger wrote:
> > > > On Sun, 13 May 2012 18:52:06 +0300
> > > > "Michael S. Tsirkin" <mst@...hat.com> wrote:
> > > > 
> > > > > +		/* Userspace may produce vectors with count greater than
> > > > > +		 * MAX_SKB_FRAGS, so we need to linearize parts of the skb
> > > > > +		 * to let the rest of data to be fit in the frags.
> > > > > +		 */
> > > > Rather than complex partial code, just go through slow path for
> > > > requests with too many frags (or for really small requests).
> > > > Creating mixed skb's seems too easy to get wrong.
> > > 
> > > I don't object in principle but macvtap has same code
> > > so seems better to stay consistent.
> > > 
> > 
> > If I remember well, code in vtap was buggy and still is.
> > 
> > Jason Wang fixes are not yet in,
> 
> They seem to be in net-next, or did I miss something?
> 
> > so maybe wait a bit, so that we dont
> > add a pile of new bugs ?
> > 
> > 

Things progress smoother upstream than out of tree
is my experience.

Also everything is guarded by a mod param in vhost
which is off by default and the name
experimental_zcopytx makes it hopefully clear there's
risk involved.

So the chance of hurting someone is imo minimal.

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