Re: [Qemu-devel] [RFC PATCH] qdev: Mark devices as non-hotpluggable by default

[Marcel Apfelbaum]

Hi Thomas,

On 19/09/2017 11:55, Thomas Huth wrote:
> Historically we've marked all devices as hotpluggable by default. However,
> most devices are not hotpluggable, and you also need a HotplugHandler to
> support these devices. So if the user tries to "device_add" or "device_del"
> such a non-hotpluggable device during runtime, either nothing really usable
> happens, or QEMU even crashes/aborts unexpectedly (see for example commit
> 84ebd3e8c7d4fe955b - "Mark diag288 watchdog as non-hotpluggable").
> So let's change this dangerous default behaviour and mark the devices as
> non-hotpluggable by default. Certain parent devices classes which are known
> as hotpluggable (e.g. PCI, USB, etc.) are marked with "hotpluggable = true",
> so that devices that are derived from these classes continue to work as
> expected.
>
> Signed-off-by: Thomas Huth <address@hidden>
> ---
>   Note: I've marked the patch as RFC since I'm not 100% sure whether I've
>   correctly identified all devices that should still be marked as hot-
>   pluggable. Feedback is welcome!
>
>   hw/core/qdev.c        | 10 ++++------
>   hw/cpu/core.c         |  1 +
>   hw/mem/nvdimm.c       |  3 +++
>   hw/mem/pc-dimm.c      |  1 +
>   hw/pci/pci.c          |  1 +
>   hw/s390x/ccw-device.c |  1 +
>   hw/scsi/scsi-bus.c    |  1 +
>   hw/usb/bus.c          |  1 +
>   8 files changed, 13 insertions(+), 6 deletions(-)
>
> diff --git a/hw/core/qdev.c b/hw/core/qdev.c
> index 606ab53..c4f1902 100644
> --- a/hw/core/qdev.c
> +++ b/hw/core/qdev.c
> @@ -1120,13 +1120,11 @@ static void device_class_init(ObjectClass *class, void 
> *data)
>       dc->realize = device_realize;
>       dc->unrealize = device_unrealize;
>   
> -    /* by default all devices were considered as hotpluggable,
> -     * so with intent to check it in generic qdev_unplug() /
> -     * device_set_realized() functions make every device
> -     * hotpluggable. Devices that shouldn't be hotpluggable,
> -     * should override it in their class_init()
> +    /*
> +     * All devices are considered as cold-pluggable by default. The devices
> +     * that are hotpluggable should override it in their class_init().
>        */
> -    dc->hotpluggable = true;
> +    dc->hotpluggable = false;

I agree with the defensive mode as long as we don't
introduce any regression...

By the way, in case a base class of a "family" of devices
would set the "hotpluggable" field to true (e.g. PCI devices),
we would still have the same problem on the specific sub-tree.
If the sub-tree is wide enough, this patch might not have the
intended impact. (we will still have the same bugs as the one
you mentioned in the commit message)

Thanks,
Marcel

>       dc->user_creatable = true;
>   }
>   
> diff --git a/hw/cpu/core.c b/hw/cpu/core.c
> index bd578ab..01660aa 100644
> --- a/hw/cpu/core.c
> +++ b/hw/cpu/core.c
> @@ -82,6 +82,7 @@ static void cpu_core_class_init(ObjectClass *oc, void *data)
>       DeviceClass *dc = DEVICE_CLASS(oc);
>   
>       set_bit(DEVICE_CATEGORY_CPU, dc->categories);
> +    dc->hotpluggable = true;
>   }
>   
>   static const TypeInfo cpu_core_type_info = {
> diff --git a/hw/mem/nvdimm.c b/hw/mem/nvdimm.c
> index 952fce5..02be9f3 100644
> --- a/hw/mem/nvdimm.c
> +++ b/hw/mem/nvdimm.c
> @@ -148,9 +148,12 @@ static MemoryRegion 
> *nvdimm_get_vmstate_memory_region(PCDIMMDevice *dimm)
>   
>   static void nvdimm_class_init(ObjectClass *oc, void *data)
>   {
> +    DeviceClass *dc = DEVICE_CLASS(oc);
>       PCDIMMDeviceClass *ddc = PC_DIMM_CLASS(oc);
>       NVDIMMClass *nvc = NVDIMM_CLASS(oc);
>   
> +    dc->hotpluggable = true;
> +
>       ddc->realize = nvdimm_realize;
>       ddc->get_memory_region = nvdimm_get_memory_region;
>       ddc->get_vmstate_memory_region = nvdimm_get_vmstate_memory_region;
> diff --git a/hw/mem/pc-dimm.c b/hw/mem/pc-dimm.c
> index 66eace5..1f78567 100644
> --- a/hw/mem/pc-dimm.c
> +++ b/hw/mem/pc-dimm.c
> @@ -443,6 +443,7 @@ static void pc_dimm_class_init(ObjectClass *oc, void *data)
>       dc->unrealize = pc_dimm_unrealize;
>       dc->props = pc_dimm_properties;
>       dc->desc = "DIMM memory module";
> +    dc->hotpluggable = true;
>   
>       ddc->get_memory_region = pc_dimm_get_memory_region;
>       ddc->get_vmstate_memory_region = pc_dimm_get_vmstate_memory_region;
> diff --git a/hw/pci/pci.c b/hw/pci/pci.c
> index 1e6fb88..8db380d 100644
> --- a/hw/pci/pci.c
> +++ b/hw/pci/pci.c
> @@ -2534,6 +2534,7 @@ static void pci_device_class_init(ObjectClass *klass, 
> void *data)
>       k->unrealize = pci_qdev_unrealize;
>       k->bus_type = TYPE_PCI_BUS;
>       k->props = pci_props;
> +    k->hotpluggable = true;
>       pc->realize = pci_default_realize;
>   }
>   
> diff --git a/hw/s390x/ccw-device.c b/hw/s390x/ccw-device.c
> index f9bfa15..d1b6e6f 100644
> --- a/hw/s390x/ccw-device.c
> +++ b/hw/s390x/ccw-device.c
> @@ -48,6 +48,7 @@ static void ccw_device_class_init(ObjectClass *klass, void 
> *data)
>       k->realize = ccw_device_realize;
>       k->refill_ids = ccw_device_refill_ids;
>       dc->props = ccw_device_properties;
> +    dc->hotpluggable = true;
>   }
>   
>   const VMStateDescription vmstate_ccw_dev = {
> diff --git a/hw/scsi/scsi-bus.c b/hw/scsi/scsi-bus.c
> index e364410..338180d 100644
> --- a/hw/scsi/scsi-bus.c
> +++ b/hw/scsi/scsi-bus.c
> @@ -2123,6 +2123,7 @@ static void scsi_device_class_init(ObjectClass *klass, 
> void *data)
>       k->realize   = scsi_qdev_realize;
>       k->unrealize = scsi_qdev_unrealize;
>       k->props     = scsi_props;
> +    k->hotpluggable = true;
>   }
>   
>   static void scsi_dev_instance_init(Object *obj)
> diff --git a/hw/usb/bus.c b/hw/usb/bus.c
> index d910f84..16701aa 100644
> --- a/hw/usb/bus.c
> +++ b/hw/usb/bus.c
> @@ -793,6 +793,7 @@ static void usb_device_class_init(ObjectClass *klass, void 
> *data)
>       k->realize  = usb_qdev_realize;
>       k->unrealize = usb_qdev_unrealize;
>       k->props    = usb_props;
> +    k->hotpluggable = true;
>   }
>   
>   static const TypeInfo usb_device_type_info = {