Re: [Qemu-devel] Discussion: redundant process during hotplug and missed process during unplug

[Michael S. Tsirkin]

On Fri, Jul 19, 2019 at 02:35:14AM +0000, Zhangbo (Oscar) wrote:
> Hi All:
> I have 2 questions about (un)hotplug on pcie-root-port.
> First Question (hotplug failure because of redundant PCI_EXP_LNKSTA_DLLLA bit 
> set):
>     during VM boot, qemu sets PCI_EXP_LNKSTA_DLLLA according to this process:
>         pcie_cap_init() -> pcie_cap_v1_fill(), 
>     even if there's no pcie device added to the VM.
>     I noticed that during hotplug, qemu also sets PCI_EXP_LNKSTA_DLLLA in 
> pcie_cap_slot_hotplug_cb().
>     It means that the bit PCI_EXP_LNKSTA_DLLLA is set TWICE.
>     why set this bit during initializing pcie-root-port? It seems unnecessary.

Makes sense.

>     The bad side of this is it causes HOTPLUG FAILURE if we boot the VM and 
> hotplug a pcie device at the same time:
>       In VM kernel，according to this bit set, it senses a PDC event, the 
> process is: 
>         pciehp_probe -> pcie_init -> pcie_init_slot -> 
> pciehp_queue_pushbutton_work. 
>     If the 2 PDC events get too close, the VM kernel will wrongly unplug the 
> device.
> Suggestion to the 1st problem:
>       Can I remove the PCI_EXP_LNKSTA_DLLLA bit set process during 
> pcie_cap_init().
> 
> Second Question (time cost too much during pcie device unplug):
>     qemu only send ABP event to VM kernel during unpluging pcie devices, VM 
> kernel receives the
>     ABP event then sleep 5s to expect a PDC event, which causes unpluging 
> devices takes too long. 
> Suggestion to the 2nd problem:
>       Can I send ABP and *PDC* events to kernet when unplug devices.

I think we should not only set PDC but also try clearing presence bit,
even though the device is actually still there and mapped into guest
memory.
Maybe we should also not send the ABP event at all.

In both cases it's necessary to test with a non-linux guest
(e.g. a couple of versions of windows) to be sure we are not breaking
anything.


-- 
MST