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

[Marcel Apfelbaum]

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
>
>