Fwd: [Qemu-devel] [RFC PATCH 0/2] Allow RedHat PCI bridges reserve more buses than necessary during init

[Alexander Bezzubikov]

2017-07-19 21:18 GMT+03:00 Konrad Rzeszutek Wilk :
> On Wed, Jul 19, 2017 at 05:14:41PM +0000, Alexander Bezzubikov wrote:
> > ср, 19 июля 2017 г. в 16:57, Konrad Rzeszutek Wilk <
>
> [e-mail]>:
> > > On Wed, Jul 19, 2017 at 04:20:12PM +0300, Aleksandr Bezzubikov wrote:
> > > > Now PCI bridges (and PCIE root port too) get a bus range number in
> > >
> > > system init,
> > > > basing on currently plugged devices. That's why when one wants to
> > >
> > > hotplug another bridge,
> > > > it needs his child bus, which the parent is unable to provide.
> > >
> > > Could you explain how you trigger this?
> >
> > I'm trying to hot plug pcie-pci bridge into pcie root port, and Linux
>
> says
> > 'cannot allocate bus number for device bla-bla'. This obviously does not
> > allow me to use the bridge at all.
> > > > The suggested workaround is to have vendor-specific capability in
>
> RedHat
> > > generic pcie-root-port
> > > > that contains number of additional bus to reserve on BIOS PCI init.
> > >
> > > But wouldn't the proper fix be for the PCI bridge to have the
>
> subordinate
> > > value be extended to fit more bus ranges?
> >
> > What do you mean? This is what I'm trying to do. Do you suppose to get
>
> rid
> > of vendor-specific cap and use original register value instead of it?
>
> I would suggest a simple fix - each bridge has a a number of bus devices
> it can use. You have up to 255 - so you split the number of northbridge
> numbers by the amount of NUMA nodes (if that is used) - so for example
> if you have 4 NUMA nodes, each bridge would cover 63 bus numbers.
>
> Meaning the root bridge would cover 0->63 bus, 64->128, and so on.
> That gives you enough space to plug in your plugged in devices
> (up to 63).
>
> And if you need sub-briges then carve out a specific range.

The problem is that we don't know at the init moment how many subbridges we
may need,
and how deep the whole device tree will be. The key moment - PCI bridge
hotplugging
needs either rescan all buses on each bridge device addition, or reserve
space in advance during BIOS init.
In this series the second way was chosen.
> > > > Aleksandr Bezzubikov (2):
> > > >   pci: add support for direct usage of bdf for capability lookup
> > > >   pci: enable RedHat pci bridges to reserve more buses
> > > >
> > > > src/fw/pciinit.c   | 12 ++++++++++--
> > > >  src/hw/pcidevice.c | 24 ++++++++++++++++++++++++
> > > >  src/hw/pcidevice.h |  1 +
> > > >  3 files changed, 35 insertions(+), 2 deletions(-)