Re: [PATCH] hw/pci/pcie: Move hot plug capability check to pre_plug callback

[Igor Mammedov <imammedo@redhat.com>]

On Mon,  1 Jun 2020 18:29:34 +0200
Julia Suvorova <jusual@redhat.com> wrote:

> Check for hot plug capability earlier to avoid removing devices attached
> during the initialization process.
> 
> Run qemu with an unattached drive:
>   -drive file=$FILE,if=none,id=drive0 \
>   -device pcie-root-port,id=rp0,slot=3,bus=pcie.0,hotplug=off
> Hotplug a block device:
>   device_add virtio-blk-pci,id=blk0,drive=drive0,bus=rp0
> If hotplug fails on plug_cb, drive0 will be deleted.
> 
> Signed-off-by: Julia Suvorova <jusual@redhat.com>
> ---
> Hard to say if it's a bug or generally acceptable behaviour, but seems like
> hotplug_handler_plug should never fail.

_unplug shouldn't fail the rest are allowed to, but it's hard to unwind
intialization cleanly to _plug stage so if it's possible to do checks
at _pre_plug time (i.e. before device's realize() is called) we should do so.

Acked-by: Igor Mammedov <imammedo@redhat.com>

> 
>  hw/pci/pcie.c | 19 +++++++++++--------
>  1 file changed, 11 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/pci/pcie.c b/hw/pci/pcie.c
> index f50e10b8fb..5b9c022d91 100644
> --- a/hw/pci/pcie.c
> +++ b/hw/pci/pcie.c
> @@ -407,6 +407,17 @@ static void pcie_cap_slot_plug_common(PCIDevice *hotplug_dev, DeviceState *dev,
>  void pcie_cap_slot_pre_plug_cb(HotplugHandler *hotplug_dev, DeviceState *dev,
>                                 Error **errp)
>  {
> +    PCIDevice *hotplug_pdev = PCI_DEVICE(hotplug_dev);
> +    uint8_t *exp_cap = hotplug_pdev->config + hotplug_pdev->exp.exp_cap;
> +    uint32_t sltcap = pci_get_word(exp_cap + PCI_EXP_SLTCAP);
> +
> +    /* Check if hot-plug is disabled on the slot */
> +    if (dev->hotplugged && (sltcap & PCI_EXP_SLTCAP_HPC) == 0) {
> +        error_setg(errp, "Hot-plug failed: unsupported by the port device '%s'",
> +                         DEVICE(hotplug_pdev)->id);
> +        return;
> +    }
> +
>      pcie_cap_slot_plug_common(PCI_DEVICE(hotplug_dev), dev, errp);
>  }
>  
> @@ -415,7 +426,6 @@ void pcie_cap_slot_plug_cb(HotplugHandler *hotplug_dev, DeviceState *dev,
>  {
>      PCIDevice *hotplug_pdev = PCI_DEVICE(hotplug_dev);
>      uint8_t *exp_cap = hotplug_pdev->config + hotplug_pdev->exp.exp_cap;
> -    uint32_t sltcap = pci_get_word(exp_cap + PCI_EXP_SLTCAP);
>      PCIDevice *pci_dev = PCI_DEVICE(dev);
>  
>      /* Don't send event when device is enabled during qemu machine creation:
> @@ -431,13 +441,6 @@ void pcie_cap_slot_plug_cb(HotplugHandler *hotplug_dev, DeviceState *dev,
>          return;
>      }
>  
> -    /* Check if hot-plug is disabled on the slot */
> -    if ((sltcap & PCI_EXP_SLTCAP_HPC) == 0) {
> -        error_setg(errp, "Hot-plug failed: unsupported by the port device '%s'",
> -                         DEVICE(hotplug_pdev)->id);
> -        return;
> -    }
> -
>      /* To enable multifunction hot-plug, we just ensure the function
>       * 0 added last. When function 0 is added, we set the sltsta and
>       * inform OS via event notification.