[Bug 1837851] Re: hv-tlbflush malfunctions on Intel host CPUs with neither EPT nor VPID (qemu-kvm)

[Thomas Huth]

The QEMU project is currently considering to move its bug tracking to
another system. For this we need to know which bugs are still valid
and which could be closed already. Thus we are setting older bugs to
"Incomplete" now.

If you still think this bug report here is valid, then please switch
the state back to "New" within the next 60 days, otherwise this report
will be marked as "Expired". Or please mark it as "Fix Released" if
the problem has been solved with a newer version of QEMU already.

Thank you and sorry for the inconvenience.


** Changed in: qemu
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1837851

Title:
  hv-tlbflush malfunctions on Intel host CPUs with neither EPT nor VPID
  (qemu-kvm)

Status in QEMU:
  Incomplete

Bug description:
  Enabling hv-tlbflush on older hosts using Intel CPUs supporting VT-x
  but neither EPT nor VPID will lead to bluescreens on the guest.

  It seems KVM only checks if EPT is available, and if it isn't it
  forcibly uses VPID. If that's *also* not available, it defaults to
  basically a no-op hypercall, though windows is expecting the TLB to be
  flushed.

  hv-tlbflush is pretty useless on machines not supporting these
  extensions anyway (only reasonably fix I can see would be to flush the
  *entire* TLB on tlbflush hypercall in KVM (i.e. a kernel fix), but
  that would remove any performance benefits), so I would suggest some
  kind of preliminary check and warning/error if hv-tlbflush is
  specified on such a host.

  All CPUs mentioned in this thread[0] are confirmed to be affected by
  the bug, and I have successfully reproduced it on an Intel Core2Duo
  E8500.

  [0] https://forum.proxmox.com/threads/windows-guest-bluescreen-with-
  proxmox-6.56053/

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