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SubjectRe: [Alacrityvm-devel] [PATCH v3 3/6] vbus: add a "vbus-proxy" bus model for vbus_driver objects
On Tue, Aug 18, 2009 at 07:51:21PM +0300, Avi Kivity wrote:
> On 08/18/2009 06:53 PM, Ira W. Snyder wrote:
>> So, in my system, copy_(to|from)_user() is completely wrong. There is no
>> userspace, only a physical system. In fact, because normal x86 computers
>> do not have DMA controllers, the host system doesn't actually handle any
>> data transfer!
>>
>
> In fact, modern x86s do have dma engines these days (google for Intel
> I/OAT), and one of our plans for vhost-net is to allow their use for
> packets above a certain size. So a patch allowing vhost-net to
> optionally use a dma engine is a good thing.
>

Yes, I'm aware that very modern x86 PCs have general purpose DMA
engines, even though I don't have any capable hardware. However, I think
it is better to support using any PC (with or without DMA engine, any
architecture) as the PCI master, and just handle the DMA all from the
PCI agent, which is known to have DMA?

>> I used virtio-net in both the guest and host systems in my example
>> virtio-over-PCI patch, and succeeded in getting them to communicate.
>> However, the lack of any setup interface means that the devices must be
>> hardcoded into both drivers, when the decision could be up to userspace.
>> I think this is a problem that vbus could solve.
>>
>
> Exposing a knob to userspace is not an insurmountable problem; vhost-net
> already allows changing the memory layout, for example.
>

Let me explain the most obvious problem I ran into: setting the MAC
addresses used in virtio.

On the host (PCI master), I want eth0 (virtio-net) to get a random MAC
address.

On the guest (PCI agent), I want eth0 (virtio-net) to get a specific MAC
address, aa:bb:cc:dd:ee:ff.

The virtio feature negotiation code handles this, by seeing the
VIRTIO_NET_F_MAC feature in it's configuration space. If BOTH drivers do
not have VIRTIO_NET_F_MAC set, then NEITHER will use the specified MAC
address. This is because the feature negotiation code only accepts a
feature if it is offered by both sides of the connection.

In this case, I must have the guest generate a random MAC address and
have the host put aa:bb:cc:dd:ee:ff into the guest's configuration
space. This basically means hardcoding the MAC addresses in the Linux
drivers, which is a big no-no.

What would I expose to userspace to make this situation manageable?

Thanks for the response,
Ira


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