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.
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.
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