Gregory Haskins wrote:
I think there is a slight disconnect here. This is *exactly* what I am
trying to do. You can of course do this many ways, and I am not denying
it could be done a different way than the path I have chosen. One
extreme would be to just slam a virtio-net specific chunk of code
directly into kvm on the host. Another extreme would be to build a
generic framework into Linux for declaring arbitrary IO types,
integrating it with kvm (as well as other environments such as lguest,
userspace, etc), and building a virtio-net model on top of that.
So in case it is not obvious at this point, I have gone with the latter
approach. I wanted to make sure it wasn't kvm specific or something
like pci specific so it had the broadest applicability to a range of
environments. So that is why the design is the way it is. I understand
that this approach is technically "harder/more-complex" than the "slam
virtio-net into kvm" approach, but I've already done that work. All we
need to do now is agree on the details ;)
virtio is already non-kvm-specific (lguest uses it) and non-pci-specific
(s390 uses it).
That said, I don't think we're bound today by the fact that we're in
userspace.
You will *always* be bound by the fact that you are in userspace. Its
purely a question of "how much" and "does anyone care". Right now,
the anwer is "a lot (roughly 45x slower)" and "at least Greg's customers
do". I have no doubt that this can and will change/improve in the
future. But it will always be true that no matter how much userspace
improves, the kernel based solution will always be faster. Its simple
physics. I'm cutting out the middleman to ultimately reach the same
destination as the userspace path, so userspace can never be equal.
If you have a good exit mitigation scheme you can cut exits by a factor
of 100; so the userspace exit costs are cut by the same factor. If you
have good copyless networking APIs you can cut the cost of copies to
zero (well, to the cost of get_user_pages_fast(), but a kernel solution
needs that too).
--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
