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?
Den
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
