[Bug 1859418] [NEW] disk driver with iothread setting hangs live migrations

[Mark Zealey]

Public bug reported:

Per report raised at https://bugzilla.redhat.com/show_bug.cgi?id=1790093

Description of problem:

A disk driver definition using iothread parameter causes live migration
with copy storage to hang during or just before the final ram sync
stage.

Interestingly, having the scsi controller as a separate iothread does
not trigger the issue.

Version-Release number of selected component (if applicable):

I can reproduce this on centos7 with qemu-ev and with centos 8:

qemu-kvm-ev-2.12.0-33.1.el7_7.4.x86_64
qemu-kvm-2.12.0-65.module_el8.0.0+189+f9babebb.5.x86_64

Steps to Reproduce:
1. Create a definition with 1 iothread on the disk image:

      <driver name='qemu' type='qcow2' iothread='1' />

2. Issue a live migrate request like: virsh migrate --live --copy-storage-all 
vm qemu+tcp://remote/system
3. Live migrate on source copies storage and then hangs at 80-99%, I guess 
during the ram copy phase.

Keeping exactly the same config but without the iothread on the disk
driver has successful migrations every time.

** Affects: qemu
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1859418

Title:
  disk driver with iothread setting hangs live migrations

Status in QEMU:
  New

Bug description:
  Per report raised at
  https://bugzilla.redhat.com/show_bug.cgi?id=1790093

  Description of problem:

  A disk driver definition using iothread parameter causes live
  migration with copy storage to hang during or just before the final
  ram sync stage.

  Interestingly, having the scsi controller as a separate iothread does
  not trigger the issue.

  Version-Release number of selected component (if applicable):

  I can reproduce this on centos7 with qemu-ev and with centos 8:

  qemu-kvm-ev-2.12.0-33.1.el7_7.4.x86_64
  qemu-kvm-2.12.0-65.module_el8.0.0+189+f9babebb.5.x86_64

  Steps to Reproduce:
  1. Create a definition with 1 iothread on the disk image:

        <driver name='qemu' type='qcow2' iothread='1' />

  2. Issue a live migrate request like: virsh migrate --live --copy-storage-all 
vm qemu+tcp://remote/system
  3. Live migrate on source copies storage and then hangs at 80-99%, I guess 
during the ram copy phase.

  Keeping exactly the same config but without the iothread on the disk
  driver has successful migrations every time.

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