Re: an issue for device hot-unplug

[Yu Zhang <yu.zhang@ionos.com>]

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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.

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
>
>

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