Re: [XenPPC] [PATCH] Enable SMP and IPIs

[Jimi Xenidis <jimix@xxxxxxxxxxxxxx>]

On Oct 24, 2006, at 9:15 PM, Amos Waterland wrote:

> On Tue, Oct 24, 2006 at 05:54:47AM -0400, Jimi Xenidis wrote:
> > On Oct 24, 2006, at 12:22 AM, Amos Waterland wrote:
> > > Note that the flurry of IPIs generated by domain creation seems to
> > > be a waste of time.  Xen on x86 doesn't actually to do anything in
> > > response to them, so I have made Xen on PPC properly deliver the
> > > event check  but then do nothing as well.  Comments?
> What do you think we should be doing in response to these event check
> IPIs?
It seems to be used to force a preemption event on a CPU.
The particular use is probably to wake CPUs napping in the idle  
domain to "raise_softirq".
However, I would think that this could have been done with a "call  
function".
>   I thought that there were hopes of improving interactivity of
> single-cpu systems ...
Not sure what your point is here. I don't see how any IPI can do that.

> > I know this is RFC, and you know I'm a bug fan of BUG(), but
> > eventually I'd expect:
> >   BUG_ON(!action)
> >   if (!action)
> >         return -ENOMEM
> I think your comments here are predicated on the assumption that BUG
> macros become noops when not compiling with debug.  That is not the  
> case
> in Xen, so I thought it was a waste of time to put in extra code.  Do
> you think it might someday not be the case?
I personally consider BUG() to be more a developer tool and in  
general should not be considered a no-return, however the Xen  
maintainers do not agree with this, but I'd like to see it reflected  
in our code.
However, you raise an interesting issue where I would have thought  
that the caller of request_irq() would check and BUG/panic. Linux  
uses this as a standard service and we are using it as a  
compatibility function for the mpic.c driver that does _not_ check  
the return value.
-JX


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