Re: Adding pcie-root-port devices via QMP apparently isn't possible

[Marcel Apfelbaum <mapfelba@redhat.com>]

Hi Thomas,


On Tue, May 18, 2021 at 3:29 PM Thomas Parrott
<thomas.parrott@canonical.com> wrote:
>
> Due to QEMU moving towards a QMP configuration mechanism and away from config file support, the LXD team are currently in the process of migrating to using QMP to add devices to VMs (so that we can support the use of QEMU 6.0).
>
> Currently we are using the `-S` flag to freeze CPU at startup, then using QMP to add NIC devices via the `device_add` command, and then using the `cont` command to start the VM guest.
>
> However we have found that it is apparently not possible to add a pcie-root-port device via QMP.
>
> Initially we tried using something like:
>
> device_add {"id":"qemu_pcie5","driver":"pcie-root-port","bus":"pcie.0","addr":"1.5","chassis":5,"multifunction":"on"}
>
> Which was a straight conversion from the current config file we use.
>
> However this gave the error:
>
> Error: Bus 'pcie.0' does not support hotplugging
>
> Then I found the `--preconfig` flag which sounded interesting, so we removed the use of `-daemonize` which prevents the use of --preconfig` and then tried adding the pcie-root-port devices in the preconfig stage.
>
> But this resulted in the error:
>
> The command 'device_add' isn't permitted in 'preconfig' state
>
> So we wondered is this a bug, or if not, what is the correct way to be adding pcie-root-ports going forward?

The pcie-root-ports cannot be hot-plugged, you will need to
pre-allocate enough pcie-root-ports for your needs.
PCI(e) devices can be hot-plugged into the pre-allocated pcie-root-ports.

Thanks,
Marcel

>
> Thanks
> Tom Parrott
>
>