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[Kashyap Chamarthy <1852437@xxxxxxxxxxxxxxxxxx>]

** Changed in: nova
       Status: Invalid => Triaged

** Changed in: nova
   Importance: Wishlist => Medium

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https://bugs.launchpad.net/bugs/1852437

Title:
  Allow ability to disable individual CPU features via
  `cpu_model_extra_flags`

Status in OpenStack Compute (nova):
  Triaged

Bug description:
  What?
  -----

  When using a custom CPU model, Nova currently allows enabling
  individual CPU flags/features via the config attribute,
  `cpu_model_extra_flags`:

      [libvirt]
      cpu_mode=custom
      cpu_model=IvyBridge
      cpu_model_extra_flags="pcid,ssbd, md-clear"

  The above only lets you enable the CPU features.  This RFE is to also
  allow _disabling_ individual CPU features.

  
  Why?
  ---

  A couple of reasons:

    - An Operator wants to generate a baseline CPU config (that facilates
      live migration) across his Compute node pool.  However, a certain
      CPU flag is causing an inteolerable performance issue for their
      guest workloads.  If the Operator isolated the problem to _that_
      specific CPU flag, then she would like to disable the flag.

    - More importantly, a specific CPU flag might trigger a CPU
      vulnerability.  In such a case, the mitigation for it could be to
      simply _disable_ the offending CPU flag.

  Allowing disabling of individual CPU flags via Nova would enable the
  above use cases.

  
  How?
  ----

  By allowing the notion of '+' / '-' to indicate whether to enable to
  disable a given CPU flag.

  E.g. if you specify the below in 'nova.conf' (on the Compute nodes):

      [libvirt]
      cpu_mode=custom
      cpu_model=IvyBridge
      cpu_model_extra_flags="+pcid,-mtrr,ssbd"

  Then, when you start an instance, Nova should generate the below XML:

      <cpu match='exact'> 
        <model fallback='forbid'>IvyBridge</model>
        <vendor>Intel</vendor>
        <feature policy='require' name='pcid'/>
        <feature policy='disable' name='mtrr'/>
        <feature policy='require' name='ssbd'/>
      </cpu>

  
  Note that the requirement to specify '+' / '-' for individual flags
  should be optional.  If neither is specified, then we should assume '+',
  and enable the feature (as shown above for the 'ssbd' flag).

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