** Changed in: nova
Status: Invalid => In Progress
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https://bugs.launchpad.net/bugs/1766208
Title:
Lift the restriction on choices for `cpu_model_extra_flags` config
attribute
Status in OpenStack Compute (nova):
In Progress
Bug description:
In the patch[*] that introduced the Nova configuration attribute
`[libvirt]/cpu_model_extra_flags`, we have restricted the choices to be
only 'PCID' -- to alleviate the immediate guest performance degradation
as a result of applying the "Meltdown" CVE fixes.
Now remove that restriction to allow adding and removing multiple CPU
flags, making way for other use cases.
Use cases:
- Ability to use 1GB huge pages with Haswell model as one use case for
extra flags:
cpu_model=Haswell-noTSX-IBRS
cpu_model_extra_flags="pdpe1gb"
- Nested Virtualization -- an operator can specify the Intel 'vmx' or
AMD 'svm' flags in the level-1 Nova guest.
[*] http://git.openstack.org/cgit/openstack/nova/commit/?h=master&;
id=6b601b7 -- libvirt: Allow to specify granular CPU feature flags
A potential example of specifying multiple CPU feature flags
------------------------------------------------------------
If you specify:
[libvirt]
cpu_mode=custom
cpu_model=IvyBridge
cpu_model_extra_flags="+pcid,-mtrr,pdpe1gb"
Then, Nova should generate the below XML:
<cpu match='exact'>
<model fallback='forbid'>IvyBridge</model>
<vendor>Intel</vendor>
<feature policy='require' name='pcid'/>
<feature policy='require' name='pdpe1gb'/>
</cpu>
The +/- for individual flags in nova.conf will be optional. If nothing
is specified, assume '+'.
You might ask: "Why would you want to remove a CPU flag though?" One
scenario for that is: An Operator wants to generate a baseline CPU
config. And a certain CPU flag is causing performance issue or other
nuisance, and if the Operator isolated the problem to _that_ specific
CPU flag, then she may want to remove the flag.
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