Re: [RFC PATCH 0/3] generic hypercall support

Avi Kivity
2009-05-07T20:17:26Z

Gregory Haskins wrote:
>> Oh yes. But don't call it dynhc - like Chris says it's the wrong
>> semantic.
>>
>> Since we want to connect it to an eventfd, call it HC_NOTIFY or
>> HC_EVENT or something along these lines. You won't be able to pass
>> any data, but that's fine. Registers are saved to memory anyway.
>>
> Ok, but how would you access the registers since you would presumably
> only be getting a waitq::func callback on the eventfd. Or were you
> saying that more data, if required, is saved in a side-band memory
> location? I can see the latter working.

Yeah. You basically have that side-band in vbus shmem (or the virtio ring).

> I can't wrap my head around
> the former.
>

I only meant that registers aren't faster than memory, since they are
just another memory location.

In fact registers are accessed through a function call (not that that
takes any time these days).

>> Just to make sure we have everything plumbed down, here's how I see
>> things working out (using qemu and virtio, use sed to taste):
>>
>> 1. qemu starts up, sets up the VM
>> 2. qemu creates virtio-net-server
>> 3. qemu allocates six eventfds: irq, stopirq, notify (one set for tx
>> ring, one set for rx ring)
>> 4. qemu connects the six eventfd to the data-available,
>> data-not-available, and kick ports of virtio-net-server
>> 5. the guest starts up and configures virtio-net in pci pin mode
>> 6. qemu notices and decides it will manage interrupts in user space
>> since this is complicated (shared level triggered interrupts)
>> 7. the guest OS boots, loads device driver
>> 8. device driver switches virtio-net to msix mode
>> 9. qemu notices, plumbs the irq fds as msix interrupts, plumbs the
>> notify fds as notifyfd
>> 10. look ma, no hands.
>>
>> Under the hood, the following takes place.
>>
>> kvm wires the irqfds to schedule a work item which fires the
>> interrupt. One day the kvm developers get their act together and
>> change it to inject the interrupt directly when the irqfd is signalled
>> (which could be from the net softirq or somewhere similarly nasty).
>>
>> virtio-net-server wires notifyfd according to its liking. It may
>> schedule a thread, or it may execute directly.
>>
>> And they all lived happily ever after.
>>
>
> Ack. I hope when its all said and done I can convince you that the
> framework to code up those virtio backends in the kernel is vbus ;)

If vbus doesn't bring significant performance advantages, I'll prefer
virtio because of existing investment.

> But
> even if not, this should provide enough plumbing that we can all coexist
> together peacefully.
>

Yes, vbus and virtio can compete on their merits without bias from some
maintainer getting in the way.

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.