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

Michael S. Tsirkin
2010-09-29T08:23:04Z

On Tue, Sep 28, 2010 at 08:24:29PM -0700, Shirley Ma wrote:
> Hello Michael,
>
> On Wed, 2010-09-15 at 07:52 -0700, Shirley Ma wrote:
> > > > Don't you think once I address vhost_add_used_and_signal update
> > > > issue, it is a simple and complete patch for macvtap TX zero copy?
> > > >
> > > > Thanks
> > > > Shirley
> > >
> > > I like the fact that the patch is simple. Unfortunately
> > > I suspect it'll stop being simple by the time it's complete :)
> >
> > I can make a try. :)
>
> I compared several approaches for addressing the issue being raised here
> on how/when to update vhost_add_used_and_signal. The simple approach I
> have found is:
>
> 1. Adding completion field in struct virtqueue;
> 2. when it is a zero copy packet, put vhost thread wait for completion
> to update vhost_add_used_and_signal;
> 3. passing vq from vhost to macvtap as skb destruct_arg;
> 4. when skb is freed for the last reference, signal vq completion
> The test results show same performance as the original patch. How do you
> think? If it sounds good to you. I will resubmit this reversion patch.
> The patch still keeps as simple as it was before. :)
>
> Thanks
> Shirley

If you look at dev_hard_start_xmit you will see a call
to skb_orphan_try which often calls the skb destructor.
So I suspect this is almost equivalent to your original patch,
and has the same correctness issue.

--
MST