Re: [kvm-ppc-devel] [PATCH 2 of 2] Add PowerPC KVM guest wait handling support

[Hollis B. <ho...@us...>]

On Tuesday 15 April 2008 11:14:52 Jerone Young wrote:
> Actually there appears to be a real problem with preempt notify in 44x.
> I had no gotten a chance to get back with you about it. But I did some
> investigation into it last week. We are following the same code paths
> (common code) as x86 for preempt initalization. But I ran some tests
> using preempt_disable() & preempt_enable() around some places where it
> would make since (places where we disable interrupts), but just using
> these functions whould cause the kernel to dump sig #11.

preempt_disable() and preempt_enable() are completely different, and I've 
called those in the past and had no problems.

> The issue we have using function kvm_vcpu_block() that it is identical
> to the code below, BUT it calls vcpu_put which then calls
> preempt_notify_unregister() if it is called it will also sig #11.

If preempt_notify_unregister() dereferences a bad pointer, that shouldn't be 
too difficult to track down.

> I'm not sure if what is going on honestly. Based on what I found it
> should "just work" as we are initializing everything like x86 (we are
> calling preempt_notify_init() in the same place). But for 44x any
> preempt notfication calls blow up. So it appears calling anything
> preempt notify related just blows up.
>
> This is a much bigger issue. I'm not sure that we honest want to be
> stuck on this for long periods of time just to have a function call in a
> place where we honestly do not absolutely need to have it at this time.
> Plus I'm no expert with these scheduling frameworks. But givin what have
> read around the net what we have now "should" work. It just doesn't.
>
> Something can come back to later. But for now we should just roll with
> the working code.

No, I don't want to commit the hack. Sorry if I didn't make that clear before.

Accessing a bad pointer doesn't seem like a "much bigger issue" to me, so if 
it's more complicated than that please elaborate.

-- 
Hollis Blanchard
IBM Linux Technology Center