Re: [PATCH v5 07/18] vfio-user: set qdev bus callbacks for remote machine

[Stefan Hajnoczi]

On Wed, Jan 19, 2022 at 04:41:56PM -0500, Jagannathan Raman wrote:
> Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
> Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
> Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
> ---
>  hw/remote/machine.c | 57 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 57 insertions(+)
> 
> diff --git a/hw/remote/machine.c b/hw/remote/machine.c
> index 220ff01aa9..221a8430c1 100644
> --- a/hw/remote/machine.c
> +++ b/hw/remote/machine.c
> @@ -22,6 +22,60 @@
>  #include "hw/pci/pci_host.h"
>  #include "hw/remote/iohub.h"
>  
> +static bool remote_machine_get_bus(const char *type, BusState **bus,
> +                                   Error **errp)
> +{
> +    ERRP_GUARD();
> +    RemoteMachineState *s = REMOTE_MACHINE(current_machine);
> +    BusState *root_bus = NULL;
> +    PCIBus *new_pci_bus = NULL;
> +
> +    if (!bus) {
> +        error_setg(errp, "Invalid argument");
> +        return false;
> +    }
> +
> +    if (strcmp(type, TYPE_PCI_BUS) && strcmp(type, TYPE_PCI_BUS)) {
> +        return true;
> +    }
> +
> +    root_bus = qbus_find_recursive(sysbus_get_default(), NULL, 
> TYPE_PCIE_BUS);
> +    if (!root_bus) {
> +        error_setg(errp, "Unable to find root PCI device");
> +        return false;
> +    }
> +
> +    new_pci_bus = pci_isol_bus_new(root_bus, type, errp);
> +    if (!new_pci_bus) {
> +        return false;
> +    }
> +
> +    *bus = BUS(new_pci_bus);
> +
> +    pci_bus_irqs(new_pci_bus, remote_iohub_set_irq, remote_iohub_map_irq,
> +                 &s->iohub, REMOTE_IOHUB_NB_PIRQS);
> +
> +    return true;
> +}

Can the user create the same PCI bus via QMP commands? If so, then this
is just a convenience that saves the extra step. Or is there some magic
that cannot be done via QMP device_add?

I'm asking because there are 3 objects involved and I'd like to
understand the lifecycle/dependencies:
1. The PCIDevice we wish to export.
2. The PCIBus with isolated address spaces that contains the PCIDevice.
3. The vfio-user server that exports a given PCIDevice.

Users can already create the PCIDevice via hotplug and the vfio-user
server via object-add. So if there's no magic they could also create the
PCI bus:
1. device_add ...some PCI bus stuff here...,id=isol-pci-bus0
2. device_add ...the PCIDevice...,bus=isol-pci-bus0,id=mydev
3. object-add x-vfio-user-server,device=mydev

Unplug would work in the reverse order.

It may be more convenient to automatically create a PCIBus when the
PCIDevice is hotplugged, but this kind of magic also has drawbacks
(hidden devices, naming collisions, etc).

> +
> +static bool remote_machine_put_bus(BusState *bus, Error **errp)
> +{
> +    PCIBus *pci_bus = NULL;
> +
> +    if (!bus) {
> +        error_setg(errp, "Invalid argument");
> +        return false;
> +    }
> +
> +    if (!object_dynamic_cast(OBJECT(bus), TYPE_PCI_BUS)) {
> +        return true;
> +    }
> +
> +    pci_bus = PCI_BUS(bus);
> +
> +    return pci_isol_bus_free(pci_bus, errp);
> +}
> +
>  static void remote_machine_init(MachineState *machine)
>  {
>      MemoryRegion *system_memory, *system_io, *pci_memory;
> @@ -56,6 +110,9 @@ static void remote_machine_init(MachineState *machine)
>                   &s->iohub, REMOTE_IOHUB_NB_PIRQS);
>  
>      qbus_set_hotplug_handler(BUS(pci_host->bus), OBJECT(s));
> +
> +    qdev_set_bus_cbs(remote_machine_get_bus, remote_machine_put_bus,
> +                     &error_fatal);
>  }
>  
>  static void remote_machine_pre_plug_cb(HotplugHandler *hotplug_dev,
> -- 
> 2.20.1
>

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