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    SubjectRe: [PATCH RFC v5] net: add PCINet driver
    On Thu, Jan 08, 2009 at 11:27:16AM -0800, Ira Snyder wrote:
    > On Thu, Jan 08, 2009 at 11:16:10AM -0800, David Miller wrote:
    > > From: Ira Snyder <iws@ovro.caltech.edu>
    > > Date: Wed, 7 Jan 2009 11:50:52 -0800
    > >
    > > > This adds support to Linux for a virtual ethernet interface which uses the
    > > > PCI bus as its transport mechanism. It creates a simple, familiar, and fast
    > > > method of communication for two devices connected by a PCI interface.
    > >
    > > Well, it looks like much more than that to me.
    > >
    > > What is this UART thing in here for?
    > >
    > > I can only assume it's meant to be used as a console port between the
    > > x86 host and the powerpc nodes.
    > >
    >
    > Exactly right. I needed it to tell the U-Boot bootloader to tftp the
    > kernel and boot the board. These boards don't keep their PCI settings
    > (assigned by the BIOS at boot) over a soft or hard reset.
    >
    > I couldn't think of a better way to get them started.
    >
    > > You haven't even mentioned this UART aspect even indirectly in the
    > > commit message.
    > >
    >
    > Sorry, I'll add something about it.
    >
    > > This just looks like yet another set of virtualization drivers
    > > to me. You could have just have easily built this using your
    > > own PCI backplane framework, and using the virtio stuff on top.
    > >
    > > And the virtio stuff has all kinds of snazzy optimizations that
    > > will likely improve your throughput, it has console drivers that
    > > distributions already probe for and attach appropriately, etc.
    > >
    > > In short I really don't like this conceptually, it can be done
    > > so much better using facilities we already have that are
    > > heavily optimized and userland understands already.
    >
    > I've had a really hard time understanding the virtio code. I haven't
    > been able to find much in the way of documentation for it. Can you point
    > me to some code that does anything vaguely similar to what you are
    > suggesting?
    >
    > Arnd Bergmann said that there is a similar driver (for different
    > hardware) that uses virtio, but it is very slow, because it doesn't use
    > the DMA controller to transfer data. I need at very minimum 40MB/sec of
    > client -> host data transfer.
    >

    Rusty, since you wrote the virtio code, can you point me at the things I
    would need to implement to use virtio over the PCI bus.

    The guests (PowerPC computers running Linux) are PCI cards in the host
    system (an Intel Pentium3-M system). The guest computers can access all
    of the host's memory. The guests provide a 1MB (movable) window into
    their memory.

    The PowerPC computers also have a DMA controller, which I've used to get
    better throughput from my driver. I have a way to create interrupts to
    both the host and guest systems.

    I've read your paper titled: "virtio: Towards a De-Facto Standard For
    Virtual I/O Devices"

    If I read that correctly, then I should implement all of the functions
    in struct virtqueue_ops appropriately, and the existing virtio drivers
    should just work. The only concern I have there is how to make guest ->
    host transfer use the DMA controller. I've done all programming of it
    from the guest kernel, using the Linux DMAEngine API.

    Are there any other implementations other than virtio_ring?

    I appreciate any input you can give. If I should be CCing someone else,
    just let me know and I'll drop you from the CC list.

    Thanks,
    Ira


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