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

[Avi Kivity]

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.

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