+++ This bug was initially created as a clone of Bug #1772032 +++

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).

--- Additional comment from Kashyap Chamarthy on 2019-11-13 14:08:45 UTC ---

Upstream blueprint: https://blueprints.launchpad.net/nova/+spec/allow-disabling-cpu-flags

--- Additional comment from Eduardo Habkost on 2019-11-13 21:39:15 UTC ---

Additional background information:

One possible mitigation for TAA (TSX Asynchronous Abort, CVE-2019-11135)[1][2] is to disable TSX using `tsx=off` in the kernel command line.

However, to be able to use `tsx=off`, customers need the ability to disable TSX in the VM CPU configurations too, otherwise VMs might become unbootable when using `tsx=off`.

[1] https://access.redhat.com/solutions/tsx-asynchronousabort
[2] https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/tsx_async_abort.html

--- Additional comment from Kashyap Chamarthy on 2021-03-08 16:19:39 UTC ---

This is merged upstream, based on these two commits:

(1) https://opendev.org/openstack/nova/commit/2e8e04a — libvirt: Don't drop CPU flags with policy='disable' from guest XML

(2) https://opendev.org/openstack/nova/commit/bcd6b42 — libvirt: Allow disabling CPU flags via `cpu_model_extra_flags`