Re: an issue for device hot-unplug

[Jinpu Wang]

Hi Yu and Laurent,

On Mon, Apr 3, 2023 at 6:59 PM Yu Zhang <yu.zhang@ionos.com> wrote:
>
> Dear Laurent,
>
> Thank you for your quick reply. We used qemu-7.1, but it is reproducible with 
> qemu from v6.2 to the recent v8.0 release candidates.
> I found that it's introduced by the commit  9323f892b39 (between v6.2.0-rc2 
> and v6.2.0-rc3).
>
> If it doesn't break anything else, it suffices to remove the line below from 
> acpi_pcihp_device_unplug_request_cb():
>
>     pdev->qdev.pending_deleted_event = true;
>
> but you may have a reason to keep it. First of all, I'll open a bug in the 
> bug tracker and let you know.
>
We opened an issue here:
https://gitlab.com/qemu-project/qemu/-/issues/1577

Regards!
Jinpu Wang
> Best regards,
> Yu Zhang
>
> On Mon, Apr 3, 2023 at 6:32 PM Laurent Vivier <lvivier@redhat.com> wrote:
>>
>> Hi Yu,
>>
>> please open a bug in the bug tracker:
>>
>> https://gitlab.com/qemu/qemu/-/issues
>>
>> It's easier to track the problem.
>>
>> What is the version of QEMU you are using?
>> Could you provide QEMU command line?
>>
>> Thanks,
>> Laurent
>>
>>
>> On 4/3/23 15:24, Yu Zhang wrote:
>> > Dear Laurent,
>> >
>> > recently we run into an issue with the following error:
>> >
>> > command '{ "execute": "device_del", "arguments": { "id": "virtio-diskX" } 
>> > }' for VM "id"
>> > failed ({ "return": {"class": "GenericError", "desc": "Device virtio-diskX 
>> > is already in
>> > the process of unplug"} }).
>> >
>> > The issue is reproducible. With a few seconds delay before hot-unplug, 
>> > hot-unplug just
>> > works fine.
>> >
>> > After a few digging, we found that the commit 9323f892b39 may incur the 
>> > issue.
>> > ------------------
>> >      failover: fix unplug pending detection
>> >
>> >      Failover needs to detect the end of the PCI unplug to start migration
>> >      after the VFIO card has been unplugged.
>> >
>> >      To do that, a flag is set in pcie_cap_slot_unplug_request_cb() and 
>> > reset in
>> >      pcie_unplug_device().
>> >
>> >      But since
>> >          17858a169508 ("hw/acpi/ich9: Set ACPI PCI hot-plug as default on 
>> > Q35")
>> >      we have switched to ACPI unplug and these functions are not called 
>> > anymore
>> >      and the flag not set. So failover migration is not able to detect if 
>> > card
>> >      is really unplugged and acts as it's done as soon as it's started. So 
>> > it
>> >      doesn't wait the end of the unplug to start the migration. We don't 
>> > see any
>> >      problem when we test that because ACPI unplug is faster than PCIe 
>> > native
>> >      hotplug and when the migration really starts the unplug operation is
>> >      already done.
>> >
>> >      See c000a9bd06ea ("pci: mark device having guest unplug request 
>> > pending")
>> >          a99c4da9fc2a ("pci: mark devices partially unplugged")
>> >
>> >      Signed-off-by: Laurent Vivier <lvivier@redhat.com 
>> > <mailto:lvivier@redhat.com>>
>> >      Reviewed-by: Ani Sinha <ani@anisinha.ca <mailto:ani@anisinha.ca>>
>> >      Message-Id: <20211118133225.324937-4-lvivier@redhat.com
>> > <mailto:20211118133225.324937-4-lvivier@redhat.com>>
>> >      Reviewed-by: Michael S. Tsirkin <mst@redhat.com 
>> > <mailto:mst@redhat.com>>
>> >      Signed-off-by: Michael S. Tsirkin <mst@redhat.com 
>> > <mailto:mst@redhat.com>>
>> > ------------------
>> > The purpose is for detecting the end of the PCI device hot-unplug. 
>> > However, we feel the
>> > error confusing. How is it possible that a disk "is already in the process 
>> > of unplug"
>> > during the first hot-unplug attempt? So far as I know, the issue was also 
>> > encountered by
>> > libvirt, but they simply ignored it:
>> >
>> > https://bugzilla.redhat.com/show_bug.cgi?id=1878659
>> > <https://bugzilla.redhat.com/show_bug.cgi?id=1878659>
>> >
>> > Hence, a question is: should we have the line below in  
>> > acpi_pcihp_device_unplug_request_cb()?
>> >
>> >     pdev->qdev.pending_deleted_event = true;
>> >
>> > It would be great if you as the author could give us a few hints.
>> >
>> > Thank you very much for your reply!
>> >
>> > Sincerely,
>> >
>> > Yu Zhang @ Compute Platform IONOS
>> > 03.04.2013
>>