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:

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 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