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

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Message-Id: <200904022243.21088.rusty@rustcorp.com.au>
Date:	Thu, 2 Apr 2009 22:43:19 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	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 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.
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