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

[Daniel P. Berrangé]

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
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