netdev - Re: [RFC v1] virtio: add virtio-over-PCI driver

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Date:	Wed, 18 Feb 2009 17:13:03 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Ira Snyder <iws@...o.caltech.edu>
Cc:	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	"Jan-Bernd Themann" <THEMANN@...ibm.com>, netdev@...r.kernel.org,
	linuxppc-dev@...abs.org
Subject: Re: [RFC v1] virtio: add virtio-over-PCI driver

On Wednesday 18 February 2009 08:54:25 Ira Snyder wrote:
> This adds support to Linux for using virtio between two computers linked by
> a PCI interface. This allows the use of virtio_net to create a familiar,
> fast interface for communication. It should be possible to use other virtio
> devices in the future, but this has not been tested.

Hi Ira,

  It's only first glance, but this looks sane.  Two things on first note:
don't restrict yourself to 32 feature bits (only PCI does this, and they're
going to have to hack when we reach feature 32).

  Secondly:
> +You will notice that the algorithm has no way of handling chains that are
> +not exactly the same on the host and guest system. Without setting any of
> +the fancier virtio_net features, this is the case.

Hmm, I think we can do slightly better than this.

How about prepending a 4 byte length on the host buffers?  Allows host to
specify length (for host->guest), and guest writes it to allow truncated
buffers on guest->host.

That won't allow you to transfer *more* than one buffersize to the host, but
you could use a different method (perhaps the 4 bytes indicates the *total*
length?).

Do 4-byte DMA's suck for some reason?

Cheers,
Rusty.
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