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

On 11/4/22 14:41, Daniel P. Berrangé wrote: > 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. that is an interesting point which I have initially missed. I have checked our investigation log and found that we have potentially made a mistake with making compatibility check with e1000 device instead of virtio, which is our default. This could be the reason. > > 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. this potentially an option, but I think that these port should be created automatically by libvirt to avoid major client rewrites. Will the option to keep the amount of spare ports in /etc/libvirt/qemu.conf work for you?