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** Changed in: linux (Ubuntu)
       Status: New => Incomplete

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  Ubuntu-5.0.0-33.35 introduces KVM regression with old Intel CPUs and
  Linux guests

Status in linux package in Ubuntu:

Bug description:
  Mostly the same info as on a related bugzilla entr[0].


  We got issues reported with old Intel CPUs and Linux guest run with
  QEMU/KVM after a recent kernel update which is based on

  I bisected this here, with following result:
  git bisect log 
  # bad: [3b931173c97b0d73f80ea55b72bb2966a246167f] UBUNTU: Ubuntu-5.0.0-33.35
  # good: [5d5a6b36e94909962297fae609bff487de3cc43a] UBUNTU: Ubuntu-5.0.0-30.32
  git bisect start '3b931173c97b0d73f80ea55b72bb2966a246167f'
  # good: [7b4f844b33969ab166800f8936beef153fab736e] net/ibmvnic: free reset 
  of removed device from queue
  git bisect good 7b4f844b33969ab166800f8936beef153fab736e
  # bad: [6c1fc88702a4f33886b44ce5b6f374893b95e369] arm64: tlb: Ensure we 
  an ISB following walk cache invalidation
  git bisect bad 6c1fc88702a4f33886b44ce5b6f374893b95e369
  # good: [e627a027b54eccc95f9e374d69aead7f1498877b] loop: Add 
  to compat ioctl
  git bisect good e627a027b54eccc95f9e374d69aead7f1498877b
  # good: [29919eff6333bc67ec580b454afdd8b49883df2f] libata/ahci: Drop PCS quirk
  for Denverton and beyond
  git bisect good 29919eff6333bc67ec580b454afdd8b49883df2f
  # good: [cb44193f94af73928f8df049ffbb6b4a0be136ae] PM / devfreq: passive: fix
  compiler warning
  git bisect good cb44193f94af73928f8df049ffbb6b4a0be136ae
  # good: [b1d479b27b26966aea931094b31864979d7f8102] scsi: implement .cleanup_rq
  git bisect good b1d479b27b26966aea931094b31864979d7f8102
  # bad: [ec15813844b05d8cbd4352c65a20e57d16f9f936] media: sn9c20x: Add MSI
  MS-1039 laptop to flip_dmi_table
  git bisect bad ec15813844b05d8cbd4352c65a20e57d16f9f936
  # good: [e83601f51a90d9739ced9ff42b6f202f8f802c72] parisc: Disable HP HSC-PCI
  Cards to prevent kernel crash
  git bisect good e83601f51a90d9739ced9ff42b6f202f8f802c72
  # good: [6d393bdf3b3f4b629070329488d3c6a3e142602b] KVM: x86: set
  ctxt->have_exception in x86_decode_insn()
  git bisect good 6d393bdf3b3f4b629070329488d3c6a3e142602b
  # bad: [208007519a7385a57b0c0a3c180142a521594876] KVM: x86: Manually calculate
  reserved bits when loading PDPTRS
  git bisect bad 208007519a7385a57b0c0a3c180142a521594876
  # first bad commit: [208007519a7385a57b0c0a3c180142a521594876] KVM: x86:
  Manually calculate reserved bits when loading PDPTRS

  Which is:

     KVM: x86: Manually calculate reserved bits when loading PDPTRS


      commit 16cfacc8085782dab8e365979356ce1ca87fd6cc upstream.

      Manually generate the PDPTR reserved bit mask when explicitly loading
      PDPTRs.  The reserved bits that are being tracked by the MMU reflect the
      current paging mode, which is unlikely to be PAE paging in the vast
      majority of flows that use load_pdptrs(), e.g. CR0 and CR4 emulation,
      __set_sregs(), etc...  This can cause KVM to incorrectly signal a bad
      PDPTR, or more likely, miss a reserved bit check and subsequently fail
      a VM-Enter due to a bad VMCS.GUEST_PDPTR.

      Add a one off helper to generate the reserved bits instead of sharing
      code across the MMU's calculations and the PDPTR emulation.  The PDPTR
      reserved bits are basically set in stone, and pushing a helper into
      the MMU's calculation adds unnecessary complexity without improving

      Oppurtunistically fix/update the comment for load_pdptrs().

      Note, the buggy commit also introduced a deliberate functional change,
      "Also remove bit 5-6 from rsvd_bits_mask per latest SDM.", which was
      effectively (and correctly) reverted by commit cd9ae5fe47df ("KVM: x86:
      Fix page-tables reserved bits").  A bit of SDM archaeology shows that
      the SDM from late 2008 had a bug (likely a copy+paste error) where it
      listed bits 6:5 as AVL and A for PDPTEs used for 4k entries but reserved
      for 2mb entries.  I.e. the SDM contradicted itself, and bits 6:5 are and
      always have been reserved.

      Fixes: 20c466b56168d ("KVM: Use rsvd_bits_mask in load_pdptrs()")
      Cc: Nadav Amit <>
      Reported-by: Doug Reiland <>
      Signed-off-by: Sean Christopherson <>
      Reviewed-by: Peter Xu <>
      Signed-off-by: Paolo Bonzini <>
      Signed-off-by: Greg Kroah-Hartman <>
      Signed-off-by: Kamal Mostafa <>
      Signed-off-by: Kleber Sacilotto de Souza <>

  This one is also included in the 4.19.81 (or more correctly, it's there since 
  v4.19.77) with commit 496cf984a60edb5534118a596613cc9971e406e8 [0] or
  upstream commit 16cfacc8085782dab8e365979356ce1ca87fd6cc [1].


  Funny thing is: I cannot reproduce this with a 5.3.7 (Eoan) kernel, which 
  includes above commit. So possible another patch is missing in the backport,
  did not find anything obvious though...

  So summary for reproducer:
  * dust of an host with old Intel CPU, e.g.: Intel Core2Duo CPU E8500 @3.16GHz
    (something else westmer, conroe or the like should work too, or if it's
     over 10 years ago. 
  * Install a Linux Distro or just boot the installer of that in a VM, I used
  Debian 9,
    as our users had issues with that but *not* with an ubuntu 19.10 VM.
  * see how it boot loops once a stable-kernel with above[0] backported
    is used on the host

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