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From:   bugzilla-daemon@bugzilla.kernel.org
To:     kvm@vger.kernel.org
Subject: [Bug 209253] Loss of connectivity on guest after important host <->
 guest traffic
Date:   Wed, 28 Oct 2020 17:12:06 +0000
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https://bugzilla.kernel.org/show_bug.cgi?id=209253

--- Comment #11 from Alex Williamson (alex.williamson@redhat.com) ---
I have reproduced, I don't know the solution yet, but I think I have a
workaround, at least for occurrences like shown in comment 1.  These seem to be
generated by the GeForce MSI quirk, which uses an ioeventfd for the re-arming
of the MSI.  Perhaps we haven't unwound out of the previous MMIO write before
another is triggered, but we should be running with IRQs disabled... I'm not
sure.  We can disable this ioeventfd in QEMU though at a slight interrupt
latency cost while further debugging.  libvirt xml changes:

<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
...
  <qemu:commandline>
    <qemu:arg value='-global'/>
    <qemu:arg value='vfio-pci.x-no-vfio-ioeventfd=on'/>
  </qemu:commandline>
</domain>

IOW, replace the first line to include the schema, otherwise libvirt will
ignore and remove the addition, then add the <qemu:commandline> stanza at the
end of the <domain> definition.  If using QEMU command line directly you can
similarly just add "-global vfio-pci.x-no-vfio-ioeventfd=on".

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