The recent "Meltdown" CVE fixes have resulted in a critical performance
penalty[*] that will impact every Nova guest with certain CPU models.
I.e. assume you have applied all the "Meltdown" CVE fixes, and performed
a cold reboot (explicit stop & start) of all Nova guests, for the
updates to take effect. Now, if any guests that are booted with certain
named virtual CPU models (e.g. "IvyBridge", "Westmere", etc), then those
guests, will incur noticeable performance degradation[*], while being
protected from the CVE itself.
To alleviate this guest performance impact, it is now important to
specify an obscure Intel CPU feature flag, 'PCID' (Process-Context ID)
-- for the virtual CPU models that don't already include it (more on
this below). To that end, this change will allow Nova to explicitly
specify CPU feature flags via a new configuration attribute,
`cpu_model_extra_flags`, e.g. in `nova.conf`:
...
[libvirt]
cpu_mode = custom
cpu_model = IvyBridge
cpu_model_extra_flags = pcid
...
NB: In the first iteration, the choices for `cpu_model_extra_flags` is
restricted to only 'pcid' (the option is case-insensitive) -- to address
the earlier mentioned guest performance degradation. A future patch
will remove this restriction, allowing to add / remove multiple CPU
feature flags, thus making way for other useful features.
Some have asked: "Why not simply hardcode the 'PCID' CPU feature flag
into Nova?" That's not graceful, and more importantly, impractical:
(1) Not every Intel CPU model has 'PCID':
- The only Intel CPU models that include the 'PCID' capability
are: "Haswell", "Broadwell", and "Skylake" variants.
- The libvirt / QEMU Intel CPU models: "Nehalem", "Westmere",
"SandyBridge", and "IvyBridge" will *not* expose the 'PCID'
capability, even if the host CPUs by the same name include it.
I.e. 'PCID' needs to be explicitly when using the said virtual
CPU models.
(2) Magically adding new CPU feature flags under the user's feet
impacts live migration.
[*] https://groups.google.com/forum/m/#!topic/mechanical-sympathy/L9mHTbeQLNU
Closes-Bug: #1750829
Change-Id: I6bb956808aa3df58747c865c92e5b276e61aff44
BluePrint: libvirt-cpu-model-extra-flags
6b601b7cf6