Subject: Re: [PATCH V5 2/6 net-next] netdevice.h: Add zero-copy flag in
 netdevice
From: Shirley Ma <mashirle@us.ibm.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: =?UTF-8?Q?Micha=C5=82_Miros=C5=82aw?= <mirqus@gmail.com>,
        Ben Hutchings <bhutchings@solarflare.com>,
        David Miller <davem@davemloft.net>,
        Eric Dumazet <eric.dumazet@gmail.com>, Avi Kivity <avi@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>, netdev@vger.kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org
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Date: Wed, 25 May 2011 15:49:40 -0700
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On Fri, 2011-05-20 at 02:41 +0300, Michael S. Tsirkin wrote:
> So the requirements are
> - data must be released in a timely fashion (e.g. unlike virtio-net
>   tun or bridge)
The current patch doesn't enable tun zero-copy. tun will copy data It's
not an issue now. We can disallow macvtap attach to bridge when
zero-copy is enabled.

> - SG support
> - HIGHDMA support (on arches where this makes sense)

This can be checked by device flags.

> - no filtering based on data (data is mapped in guest)

> - on fast path no calls to skb_copy, skb_clone, pskb_copy,
>   pskb_expand_head as these are slow

Any calls to skb_copy, skb_clone, pskb_copy, pskb_expand_head will do a
copy. The performance should be the same as none zero-copy case before.
I have done/tested the patch V6, will send it out for review tomorrow.

I am looking at where there are some cases, skb remains the same for
filtering.

> First 2 requirements are a must, all other requirements
> are just dependencies to make sure zero copy will be faster
> than non zero copy.
> Using a new feature bit is probably the simplest approach to
> this. macvtap on top of most physical NICs most likely works
> correctly so it seems a bit more work than it needs to be,
> but it's also the safest one I think ... 

For "macvtap/vhost zero-copy" we can use SG & HIGHDMA to enable it, it
looks safe to me once patching skb_copy, skb_clone, pskb_copy,
pskb_expand_head.

To extend zero-copy in other usages, we can have a new feature bit
later.

Is that reasonable?

Thanks
Shirley


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