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[Luis Henriques <luis.henriques@xxxxxxxxxxxxx>]

** Also affects: linux (Ubuntu Xenial)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1644394

Title:
  Botched backport breaks level triggered EOIs in QEMU guests with
  --machine kernel_irqchip=split

Status in linux package in Ubuntu:
  Confirmed
Status in linux source package in Xenial:
  New

Bug description:
  The port of KVM's HyperV SynIC patches (and other HyperV related KVM
  Patches) broke QEMU's --machine kernel_irqchip=split for Ubuntu Xenial
  on x86. Guests hang in early boot.

  Ubuntu Xenial dropped the upstream patch
  abdb080f7ac8a85547f5e0246362790043bbd3f2 while backporting. The result
  is that patch a6767645c9da8c0e91ebbb47677d933d6b378638 in Ubuntu's
  tree clobbers the definition of kvm_arch_irq_routing_update, which was
  renamed in abdb080f7..., and drops the IOAPIC scan request as a
  result. [Aside: this also results in kvm_hv_irq_routing_update being
  called outside irq_lock].

  The result of this is that the EOI exit bitmaps are not updated when
  the table is updated, and no IOAPIC EOIs go to userspace.

  The fix is backporting abdb080f7ac8a85547f5e0246362790043bbd3f2, and
  defining kvm_arch_post_irq_routing_update with the old definition of
  kvm_arch_port_irq_routing_update. I've attached a patch that does
  this.

  Repro: The KVM-Unit-test's IOAPIC test should fail with QEMU and
  --machine kernel_irqchip=split on 4.4.0-36. (Caveat: I actually
  haven't tried this myself, but the theory is there.)

  Security Implications: I believe this is not a security issue.
  kvm_hv_irq_routing_update only moved outside the lock, not outside the
  srcu critical section. If the update were outside the critical
  section, this would be a racy use-after-free.

  Versioning information: I cloned the tree at
  git://kernel.ubuntu.com/ubuntu/ubuntu-xenial.git and checked out the
  tag 'Ubuntu-4.4.0-36.55'. All of the tests were done on a build of the
  4.4.0-36 kernel. Later versions also have the issue (checked
  'Ubuntu-4.4.0-49.70'). The issue was introduced in
  'Ubuntu-4.4.0-32.51'.

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