netdev - Re: [RFC PATCH 00/17] virtual-bus

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Date:	Thu, 2 Apr 2009 18:10:21 +0300
From:	"Michael S. Tsirkin" <m.s.tsirkin@...il.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Gregory Haskins <ghaskins@...ell.com>, Avi Kivity <avi@...hat.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	anthony@...emonkey.ws, andi@...stfloor.org,
	linux-kernel@...r.kernel.org, agraf@...e.de, pmullaney@...ell.com,
	pmorreale@...ell.com, netdev@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC PATCH 00/17] virtual-bus

On Thu, Apr 02, 2009 at 10:43:19PM +1030, Rusty Russell wrote:
> On Thursday 02 April 2009 21:36:07 Gregory Haskins wrote:
> > You do not need to know when the packet is copied (which I currently
> > do).  You only need it for zero-copy (of which I would like to support,
> > but as I understand it there are problems with the reliability of proper
> > callback (i.e. skb->destructor).
> 
> But if you have a UP guest, there will *never* be another packet in the queue
> at this point, since it wasn't running.
> 
> As Avi said, you can do the processing in another thread and go back to the
> guest; lguest pre-virtio did a hacky "weak" wakeup to ensure the guest ran
> again before the thread did for exactly this kind of reason.
> 
> While Avi's point about a "powerful enough userspace API" is probably valid,
> I don't think it's going to happen.  It's almost certainly less code to put a
> virtio_net server in the kernel, than it is to create such a powerful
> interface (see vringfd & tap).  And that interface would have one user in
> practice.
> 
> So, let's roll out a kernel virtio_net server.  Anyone?
> Rusty.

BTW, whatever approach is chosen, to enable zero-copy transmits, it seems that
we still must add tracking of when the skb has actually been transmitted, right?

Rusty, I think this is what you did in your patch from 2008 to add destructor
for skb data ( http://kerneltrap.org/mailarchive/linux-netdev/2008/4/18/1464944 ):
and it seems that it would make zero-copy possible - or was there some problem with
that approach? Do you happen to remember?

-- 
MST
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