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


From: fangying
Subject: Re: [Qemu-devel] Discussion: redundant process during hotplug and missed process during unplug
Date: Sat, 20 Jul 2019 15:24:46 +0800
User-agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0


Hi Michael,
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´╝îaccording 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().


We raise this qeustion here because we find out that if the pcie ext capability PCI_EXP_LNKSTA_DLLLA is set by default, linux kernel may try to send PCI_EXP_HP_EV_PDC event after boot-up. When we do virtio device hotplug during the processing of PCI_EXP_HP_EV_PDC event (pciehp_ist =>pciehp_handle_presence_or_link_change => pciehp_enable_slot) the device may be accidently powered down because the power state detected is ON_STATE.

Kernel sends PCI_EXP_HP_EV_PDC event when it tries to probe the
pcie-root-probe, i.e:
pciehp_probe => pciehp_check_presence => pciehp_card_present_or_link_active => pciehp_check_link_active pciehp_check_link_active returns true if PCI_EXP_LNKSTA_DLLLA Cap is presented.

We are going send a patch to have PCI_EXP_LNKSTA_DLLLA Cap removed to fix the problem here.
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.


Thanks to your opinion, we will try to send PCI_EXP_HP_EV_PDC event instead of the PCI_EXP_HP_EV_PDC event at device unplug and do some Windows guest compatibility test.
We will reply later.




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