Re: [PATCH] qemu: add pcie-to-pci-bridge for q35 machines

On Fri, Nov 04, 2022 at 01:03:52PM +0100, Peter Krempa wrote: > On Fri, Nov 04, 2022 at 16:43:00 +0600, Oleg Vasilev wrote: >> Hotplugging PCI devices on pc-i440fx machines is supported without >> additional configuration. On q35, pcie-to-pci-bridge needs to be added >> prior to the machine startup in order to support hotplug [1]. >> >> The idea is to support the same workflow for creating VMs in q35, as was >> in pc. Namely, do not require additional configuration when hotplugging >> is needed. Otherwise, all libvirt clients need to be updated which there >> are a lot and they are maintained by different parties. >> >> Instead, a pcie-to-pci-bridge better be created by default, so that PCI >> slots are readily available. Might be a good idea to make it configurable >> in the future. The goal of q35 is to provide a PCI-e topology though, and we want devices to be exposed as PCI-e endpoints, with no legacy PCI usage by default. When a guest XML is provided for cold boot, libvirt will ensure that all devices are placed onto the PCI-e bus via pcie-root-ports With this proposal hotplugging a device will cause it to be placed on a PCI bus instead, if there are no spare pcie-root-port available. This behavioural difference between cold plug and hot plug is very undesirable IMHO, as it is counter to our goal of avoiding legacy PCI on PCI-e machines.

If a mgmt app wants to enable this mis-matched behaviour they can add a pcie-to-pci-bridge when initially cold booting the VM, but we shouldn't add one by default IMHO. We really recommend that apps just pre-populate 10-30 pcie-root-port devices when creating a VM, so there is plenty of space for hotplugging devices later.

>> Previously there was a pci-bridge present by default, but was removed in >> d5fb8f4564 (qemu: don't add pci-bridge to Q35/arm domains unless it's needed, 2016-04-22) With regards, Daniel