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
In-Reply-To: <20110526084916.GA17928@redhat.com>
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Date: Thu, 26 May 2011 08:27:46 -0700
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On Thu, 2011-05-26 at 11:49 +0300, Michael S. Tsirkin wrote:
> On Wed, May 25, 2011 at 03:49:40PM -0700, Shirley Ma wrote:
> > 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.
> 
> Attach macvtap to a tun device though. Or e.g. veth device ...
> So there should be so generic way to disable zerocopy.
> It can either be a whitelist or a blacklist.
> > 
> > > - SG support
> > > - HIGHDMA support (on arches where this makes sense)
> > 
> > This can be checked by device flags.
> 
> OK, but pls note that SG can get turned off dynamically.
> 
> > > - 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'm guessing a copy is cheaper than get_user_pages+copy+put_page.
> But maybe not by much. Care checking that?

That's I have done already. Patch is going out for review.

> > 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.
> 
> To reliably filter on data I think we'll need to copy it first,
> otherwise
> guest can change it. Most filters only look at the header though.
> 
> > > 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
> 
> Is the problem is extra work needed to extend feature bits?

There is no problem to use it, Mahesh is working on this patch. I just
want to remove macvtap/vhost zero-copy patch dependency.

Thanks
Shirley

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