Re: [PATCH/PFC 0/2] s390 host support

[Carsten Otte <cotte-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org>]

Avi Kivity wrote:
> If the eventfd patchset is merged, then file descriptors will become the 
> standard Linux handle type, and poll (or rather, epoll) will become the 
> standard way of waiting for something to happen.  But of course if you 
> come up with something better we'll use that.
Triggered by this discussion, I have spent quite some time thinking 
about signaling, idle cpus, interrupts, signal processor (read: IPI) 
and such lately. It has become clear to me, that sleeping in userspace 
has been a bad design point I've made. The way kvm deals with this 
(idle cpu thread sleeps interruptible in kernel) is clearly preferable.

Our sie system call has quite a complex userspace interface that 
allows the user to modify various bits and pieces of our virtual cpu 
control block. All this is needed only, because we do a lot of wrong 
things in userspace. Like signal processor (read: IPI). I will go 
ahead and put these things into our kernel module. That should 
simplify our user<->kernel interface a lot.
One problem is, that we need to inject interrupts from userland. This 
requires waking up idle CPUs. I want to try how it comes out with a 
new system call for irqs rather then using tkill(). We could have the 
kernel choose the vcpu that is enabled for this interrupt for example. 
And the kernel can do optimizations like sending irqs to idle cpus 
preferably. The user could supply a CPU mask that specifies what CPUs 
come into question for the irq.

Another neat advantage of moving SIE specifics into the kernel module 
is, that our userspace will be left with device drivers only. We can 
then put those into kvm/qemu or switch to other paravirtual device 
drivers and discard our userspace code.

I believe once we've changed that, merging with kvm on both kernel and 
user side should become easier then it is today.

so long,
Carsten

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