Reviewed: https://review.openstack.org/534384 Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=6b601b7cf6e7f23077f428353a3a4e81084eb3a1 Submitter: Zuul Branch: master
commit 6b601b7cf6e7f23077f428353a3a4e81084eb3a1 Author: Kashyap Chamarthy <[email protected]> Date: Tue Jan 16 17:56:51 2018 +0100 libvirt: Allow to specify granular CPU feature flags 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 ** Changed in: nova Status: In Progress => Fix Released -- You received this bug notification because you are a member of Yahoo! Engineering Team, which is subscribed to OpenStack Compute (nova). https://bugs.launchpad.net/bugs/1750829 Title: RFE: libvirt: Add ability to configure extra CPU flags for named CPU models Status in OpenStack Compute (nova): Fix Released Status in OpenStack Compute (nova) pike series: Confirmed Status in OpenStack Compute (nova) queens series: Confirmed Bug description: Motivation ---------- The recent "Meltdown" CVE fixes resulted in critical performance penalty, From here[*]: [...] However, in examining both the various fixes rolled out in actual Linux distros over the past few days and doing some very informal surveying of environments I have access to, I discovered that the PCID ["process-context identifiers"] processor feature, which used to be a virtual no-op, is now a performance AND security critical item.[...] So if a Nova user has applied all the "Meltdown" CVE fixes, and is using a named CPU model (like "IvyBridge", or "Westmere" — which specifically lack the said obscure "PCID" feature) they will incur severe performance degradation[*]. Note that some of Intel *physical* CPUs themselves include the 'pcid' CPU feature flag; but the named CPU models provided by libvirt & QEMU lack that flag — hence we explicitly specify it for virtual CPUs via the following proposed config attribute. [*] https://groups.google.com/forum/m/#!topic/mechanical- sympathy/L9mHTbeQLNU Proposed change --------------- Modify Nova's libvirt driver such that it will be possible to set granular CPU feature flags for named CPU models. E.g. to explicitly specify the 'pcid' feature flag with Intel IvyBridge CPU model, set the following in /etc/nova.conf: ... [libvirt] cpu_model=IvyBridge cpu_model_extra_flags="pcid" ... The list of known CPU feature flags ('vmx', 'xtpr', 'pcid', et cetera) can be found in /usr/share/libvirt/cpu_map.xml. Note that before specifying extra CPU feature flags, one should check if the named CPU models (provided by libvirt) already include the said flags. E.g. the 'Broadwell', 'Haswell-noTSX' named CPU models provided by libvirt already provides the 'pcid' CPU feature flag. Other use cases --------------- - Nested Virtualization — an operator can specify the Intel 'vmx' or AMD 'svm' flags in the level-1 guest (i.e. the guest hypervisor) - Ability to use 1GB huge pages with Haswell model as one use case for extra flags (thanks: Daniel Berrangé, for mentioning this scenario): cpu_model_extra_flags=Haswell cpu_model_extra_flags="pdpe1gb" To manage notifications about this bug go to: https://bugs.launchpad.net/nova/+bug/1750829/+subscriptions -- Mailing list: https://launchpad.net/~yahoo-eng-team Post to : [email protected] Unsubscribe : https://launchpad.net/~yahoo-eng-team More help : https://help.launchpad.net/ListHelp
