[SeaBIOS] hotplug failure issue on pci-bridge

Laszlo Ersek lersek at redhat.com
Tue Jul 17 15:42:06 CEST 2018

On 07/16/18 11:45, Liu, Jing2 wrote:
> Hi Laszlo,
> On 7/12/2018 3:29 PM, Laszlo Ersek wrote:
>> On 07/12/18 07:43, Liu, Jing2 wrote:
>>> Yep, thanks for the advice.
>>> But hotplugging on pci-bridge is the actual use case
>>> request so we would better solve and fix this.
>> You can cold-plug a PCI Express Root Port in the Q35 root complex
>> (pcie.0), reserving the MMIO resources you want, cold-plug a PCIE-PCI
>> Bridge in that root port, and then hot-plug the desired endpoint into
>> that PCIE-PCI Bridge. 
> I'm trying this. But actual results show that,
> when pcie-pci-bridge has no coldplug device, it shows all NONE for each
> windows.
> 01:00.0 PCI bridge: Red Hat, Inc. Device 000e (prog-if 00 [Normal decode])
>         I/O behind bridge: None
>         Memory behind bridge: None
>         Prefetchable memory behind bridge: None

Can you check /proc/iomem, and dmesg?

What is your exact QEMU command line?

BTW, there's also <https://bugzilla.redhat.com/show_bug.cgi?id=1536147>.
It might be relevant here.

(I haven't personally tested SeaBIOS in the hotplug scenario at hand,
but it's been my understanding that, as long as you cold-plug the
PCIe-PCI bridge itself, RHBZ#1536147 shouldn't apply, and the hotplug
into the bridge should just work. Personally I've only tested the same
with OVMF only.)

Adding Marcel and Alexander to the thread (likely belatedly; sorry about


> Only if I cold plug some device (e.g. e1000) under it, and then hotplug
> another device might be successful.
> BTW, I open the guest kernel config: CONFIG_PCI_REALLOC_ENABLE_AUTO=y,
> but it doesn't work.
> I'm not sure if there are some other issues I forgot?
> Jing
> This is one of the exact examples that
>> "docs/pcie_pci_bridge.txt" provides. (The other example is when the
>> PCIE-PCI bridge itself is hot-plugged into the root port, for which bus
>> number reservation is necessary too, at the root port level.)
>> If you want more than that, e.g. do something similar on i440fx, that
>> will take QEMU work as well, not just SeaBIOS.
>> Laszlo

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