Re: [Qemu-devel] [RFC PATCH v2 0/4] Allow RedHat PCI bridges reserve more buses than necessary during init

[Michael S. Tsirkin]

On Wed, Jul 26, 2017 at 07:22:42PM +0300, Marcel Apfelbaum wrote:
> On 26/07/2017 18:20, Laszlo Ersek wrote:
> > On 07/26/17 08:48, Marcel Apfelbaum wrote:
> > > On 25/07/2017 18:46, Laszlo Ersek wrote:
> > 
> > [snip]
> > 
> > > > (2) Bus range reservation, and hotplugging bridges. What's the
> > > > motivation? Our recommendations in "docs/pcie.txt" suggest flat
> > > > hierarchies.
> > > > 
> > > 
> > > It remains flat. You have one single PCIE-PCI bridge plugged
> > > into a PCIe Root Port, no deep nesting.
> > > 
> > > The reason is to be able to support legacy PCI devices without
> > > "committing" with a DMI-PCI bridge in advance. (Keep Q35 without)
> > > legacy hw.
> > > 
> > > The only way to support PCI devices in Q35 is to have them cold-plugged
> > > into the pcie.0 bus, which is good, but not enough for expanding the
> > > Q35 usability in order to make it eventually the default
> > > QEMU x86 machine (I know this is another discussion and I am in
> > > minority, at least for now).
> > > 
> > > The plan is:
> > > Start Q35 machine as usual, but one of the PCIe Root Ports includes
> > > hints for firmware needed t support legacy PCI devices. (IO Ports range,
> > > extra bus,...)
> > > 
> > > Once a pci device is needed you have 2 options:
> > > 1. Plug a PCIe-PCI bridge into a PCIe Root Port and the PCI device
> > >     in the bridge.
> > > 2. Hotplug a PCIe-PCI bridge into a PCIe Root Port and then hotplug
> > >     a PCI device into the bridge.
> > 
> 
> Hi Laszlo,
> 
> > Thank you for the explanation, it makes the intent a lot clearer.
> > 
> > However, what does the hot-pluggability of the PCIe-PCI bridge buy us?
> > In other words, what does it buy us when we do not add the PCIe-PCI
> > bridge immediately at guest startup, as an integrated device?
> >  > Why is it a problem to "commit" in advance? I understand that we might
> > not like the DMI-PCI bridge (due to it being legacy), but what speaks
> > against cold-plugging the PCIe-PCI bridge either as an integrated device
> > in pcie.0 (assuming that is permitted), or cold-plugging the PCIe-PCI
> > bridge in a similarly cold-plugged PCIe root port?
> > 
> 
> We want to keep Q35 clean, and for most cases we don't want any
> legacy PCI stuff if not especially required.

BTW, what are the PCI devices that we actually need?

-- 
MST