[Ksummit-2012-discuss] PCI breakout session

Michael S. Tsirkin mst at redhat.com
Mon Jun 18 08:08:34 UTC 2012

On Mon, Jun 18, 2012 at 08:41:43AM +0100, James Bottomley wrote:
> On Mon, 2012-06-18 at 07:42 +1000, Benjamin Herrenschmidt wrote:
> > (snip general approval)
> > 
> > So folks, what should be the format ? Do we organize a mini-summit
> > proper with an agenda or we just breakout during KS itself ?
> > 
> > Ted, does the venue has facilities for us to do something maybe the day
> > before ? Or we just do it informally in the corridors ? :-)
> > 
> > I think the main thing to discuss is rework & cleanup of hotplug which
> > is a large impact (PCI boot code as well) if we want to do it properly,
> > which means having essentially the same code path overall to deal with
> > normal PCI probe and hotplug.
> > 
> > The main issue I see in term of infrastructure is how we rely on
> > initcall "levels" and ordering of things at boot to do things like
> > probe, then assign resources, then fixups, then bind drivers. It's
> > fragile and doesn't work properly with hotplug, meaning that hotplug
> > uses a subtely different ordering (and totally fails to call the final
> > fixups).
> > 
> > The "right" approach is to probably move the resource assignment to
> > between the initial scan pass and the "adding" of devices (in our PCI
> > stack terminology, adding -> register with the driver model), and
> > naturally have the final fixups called right before the latter. But this
> > will have some sort of impact on all archs so we probably want to do
> > quite a bit of code auditing first.

An interesting related problem is hot-plugging complex bridged hierarchies - esoteric
on real hardware but easy and sometimes useful in a VM.
It would seem that at least for that space, adding support
for moving resources around could be useful. For things like IO it
only seems possible to do with cooperation from the drivers - and at least for
VMs that's not such a big deal as we only emulate a handful of devices.
Or do we need to try and solve it generally straight away?

> > 
> > Of course that's scary since PCI is so prone to regressions, especially
> > on x86 ...
> > 
> > I have some specific issues with resource allocation on bridges that
> > segment the MMIO space in interesting ways (for error handling) that I
> > want to discuss and get feedback on how to best deal with.
> > 
> > Do we start writing an agenda ?
> Could we do it on the second mini-summit day?  I suspect most of the
> arch maintainers will want to be there, since PCI is bound into all our
> architectures in some quite unique ways.
> James


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