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:
- Plug a PCIe-PCI bridge into a PCIe Root Port and the PCI device in the bridge.
- 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?