CPU model configuration for QEMU/KVM on x86 hosts
With the various CPU hardware vulnerabilities reported this year, guest CPU configuration is now a security critical task. This blog post contains content I’ve written that is on its way to become part of the QEMU documentation.
QEMU / KVM virtualization supports two ways to configure CPU models
- Host passthrough
- This passes the host CPU model features, model, stepping, exactly to the guest. Note that KVM may filter out some host CPU model features if they cannot be supported with virtualization. Live migration is unsafe when this mode is used as libvirt / QEMU cannot guarantee a stable CPU is exposed to the guest across hosts. This is the recommended CPU to use, provided live migration is not required.
- Named model
- QEMU comes with a number of predefined named CPU models, that typically refer to specific generations of hardware released by Intel and AMD. These allow the guest VMs to have a degree of isolation from the host CPU, allowing greater flexibility in live migrating between hosts with differing hardware.
In both cases, it is possible to optionally add or remove individual CPU features, to alter what is presented to the guest by default.
Libvirt supports a third way to configure CPU models known as “Host model”. This uses the QEMU “Named model” feature, automatically picking a CPU model that is similar the host CPU, and then adding extra features to approximate the host model as closely as possible. This does not guarantee the CPU family, stepping, etc will precisely match the host CPU, as they would with “Host passthrough”, but gives much of the benefit of passthrough, while making live migration safe.
Recommendations for KVM CPU model configuration on x86 hosts
The information that follows provides recommendations for configuring CPU models on x86 hosts. The goals are to maximise performance, while protecting guest OS against various CPU hardware flaws, and optionally enabling live migration between hosts with hetergeneous CPU models.
Preferred CPU models for Intel x86 hosts
The following CPU models are preferred for use on Intel hosts. Administrators / applications are recommended to use the CPU model that matches the generation of the host CPUs in use. In a deployment with a mixture of host CPU models between machines, if live migration compatibility is required, use the newest CPU model that is compatible across all desired hosts.
Skylake-ServerSkylake-Server-IBRS- Intel Xeon Processor (Skylake, 2016)
Skylake-ClientSkylake-Client-IBRS- Intel Core Processor (Skylake, 2015)
BroadwellBroadwell-IBRSBroadwell-noTSXBroadwell-noTSX-IBRS- Intel Core Processor (Broadwell, 2014)
HaswellHaswell-IBRSHaswell-noTSXHaswell-noTSX-IBRS- Intel Core Processor (Haswell, 2013)
IvyBridgeIvyBridge-IBRS- Intel Xeon E3-12xx v2 (Ivy Bridge, 2012)
SandyBridgeSandyBridge-IBRS- Intel Xeon E312xx (Sandy Bridge, 2011)
WestmereWestmere-IBRS- Westmere E56xx/L56xx/X56xx (Nehalem-C, 2010)
NehalemNehalem-IBRS- Intel Core i7 9xx (Nehalem Class Core i7, 2008)
Penryn- Intel Core 2 Duo P9xxx (Penryn Class Core 2, 2007)
Conroe- Intel Celeron_4x0 (Conroe/Merom Class Core 2, 2006)
Important CPU features for Intel x86 hosts
The following are important CPU features that should be used on Intel x86 hosts, when available in the host CPU. Some of them require explicit configuration to enable, as they are not included by default in some, or all, of the named CPU models listed above. In general all of these features are included if using “Host passthrough” or “Host model”.
pcid- Recommended to mitigate the cost of the Meltdown (CVE-2017-5754) fix. Included by default in Haswell, Broadwell & Skylake Intel CPU models. Should be explicitly turned on for Westmere, SandyBridge, and IvyBridge Intel CPU models. Note that some desktop/mobile Westmere CPUs cannot support this feature.
spec-ctrl- Required to enable the Spectre (CVE-2017-5753 and CVE-2017-5715) fix, in cases where retpolines are not sufficient. Included by default in Intel CPU models with -IBRS suffix. Must be explicitly turned on for Intel CPU models without -IBRS suffix. Requires the host CPU microcode to support this feature before it can be used for guest CPUs.
ssbd- Required to enable the CVE-2018-3639 fix. Not included by default in any Intel CPU model. Must be explicitly turned on for all Intel CPU models. Requires the host CPU microcode to support this feature before it can be used for guest CPUs.
pdpe1gb- Recommended to allow guest OS to use 1GB size pages.Not included by default in any Intel CPU model. Should be explicitly turned on for all Intel CPU models. Note that not all CPU hardware will support this feature.
Preferred CPU models for AMD x86 hosts
The following CPU models are preferred for use on Intel hosts. Administrators / applications are recommended to use the CPU model that matches the generation of the host CPUs in use. In a deployment with a mixture of host CPU models between machines, if live migration compatibility is required, use the newest CPU model that is compatible across all desired hosts.
EPYCEPYC-IBPB- AMD EPYC Processor (2017)
Opteron_G5- AMD Opteron 63xx class CPU (2012)
Opteron_G4- AMD Opteron 62xx class CPU (2011)
Opteron_G3- AMD Opteron 23xx (Gen 3 Class Opteron, 2009)
Opteron_G2- AMD Opteron 22xx (Gen 2 Class Opteron, 2006)
Opteron_G1- AMD Opteron 240 (Gen 1 Class Opteron, 2004)
Important CPU features for AMD x86 hosts
The following are important CPU features that should be used on AMD x86 hosts, when available in the host CPU. Some of them require explicit configuration to enable, as they are not included by default in some, or all, of the named CPU models listed above. In general all of these features are included if using “Host passthrough” or “Host model”.
ibpb- Required to enable the Spectre (CVE-2017-5753 and CVE-2017-5715) fix, in cases where retpolines are not sufficient. Included by default in AMD CPU models with -IBPB suffix. Must be explicitly turned on for AMD CPU models without -IBPB suffix. Requires the host CPU microcode to support this feature before it can be used for guest CPUs.
virt-ssbd- Required to enable the CVE-2018-3639 fix. Not included by default in any AMD CPU model. Must be explicitly turned on for all AMD CPU models. This should be provided to guests, even if amd-ssbd is also provided, for maximum guest compatibility. Note for some QEMU / libvirt versions, this must be force enabled when when using “Host model”, because this is a virtual feature that doesn’t exist in the physical host CPUs.
amd-ssbd- Required to enable the CVE-2018-3639 fix. Not included by default in any AMD CPU model. Must be explicitly turned on for all AMD CPU models. This provides higher performance than virt-ssbd so should be exposed to guests whenever available in the host. virt-ssbd should none the less also be exposed for maximum guest compatability as some kernels only know about virt-ssbd.
amd-no-ssb- Recommended to indicate the host is not vulnerable CVE-2018-3639. Not included by default in any AMD CPU model. Future hardware genarations of CPU will not be vulnerable to CVE-2018-3639, and thus the guest should be told not to enable its mitigations, by exposing amd-no-ssb. This is mutually exclusive with virt-ssbd and amd-ssbd.
pdpe1gb- Recommended to allow guest OS to use 1GB size pages. Not included by default in any AMD CPU model. Should be explicitly turned on for all AMD CPU models. Note that not all CPU hardware will support this feature.
Default x86 CPU models
The default QEMU CPU models are designed such that they can run on all hosts. If an application does not wish to do perform any host compatibility checks before launching guests, the default is guaranteed to work.
The default CPU models will, however, leave the guest OS vulnerable to various CPU hardware flaws, so their use is strongly discouraged. Applications should follow the earlier guidance to setup a better CPU configuration, with host passthrough recommended if live migration is not needed.
qemu32qemu64- QEMU Virtual CPU version 2.5+ (32 & 64 bit variants). qemu64 is used for x86_64 guests and qemu32 is used for i686 guests, when no -cpu argument is given to QEMU, or no <cpu> is provided in libvirt XML.
Other non-recommended x86 CPUs
The following CPUs models are compatible with most AMD and Intel x86 hosts, but their usage is discouraged, as they expose a very limited featureset, which prevents guests having optimal performance.
kvm32kvm64- Common KVM processor (32 & 64 bit variants). Legacy models just for historical compatibility with ancient QEMU versions.
486athlonphenomcoreduocore2duon270pentiumpentium2pentium3- Various very old x86 CPU models, mostly predating the introduction of hardware assisted virtualization, that should thus not be required for running virtual machines.
Syntax for configuring CPU models
The example below illustrate the approach to configuring the various CPU models / features in QEMU and libvirt
QEMU command line
- Host passthrough
-
$ qemu-system-x86_64 -cpu host
With feature customization:
$ qemu-system-x86_64 -cpu host,-vmx,...
- Named CPU models
-
$ qemu-system-x86_64 -cpu Westmere
With feature customization:
$ qemu-system-x86_64 -cpu Westmere,+pcid,...
Libvirt guest XML
- Host passthrough
-
<cpu mode='host-passthrough'/>
With feature customization:
<cpu mode='host-passthrough'> <feature name="vmx" policy="disable"/> ... </cpu> - Host model
-
<cpu mode='host-model'/>
With feature customization:
<cpu mode='host-model'> <feature name="vmx" policy="disable"/> ... </cpu> - Named model
-
<cpu mode='custom'> <model>Westmere</model> </cpu>With feature customization:
<cpu mode='custom'> <model>Westmere</model> <feature name="pcid" policy="require"/> ... </cpu>
I hope I’m terribly misunderstanding something, but do we have insecurity by default? If we are then this is a dangerous practice, users should not be expected to know all this and manually force all these switches. As a user you’d think it’s disabled for very good reasons and should be left alone, but if there is none why are they disabled?
Thanks for the writeup nonetheless, it was a interesting read!
Hi Dan,
<model name=”Westmere”/> doesn’t seem like the right syntax, it should be <model>Westmere</model>.
Is it possible to use default models and include the mitigations for the different vulnerabilities? Something like kvm64 + spec-ctrl …
Yes, it is possible to use kvm64,+spec-ctrl but I won’t not really advise doing that for a number of reasons.
First as mentioned kvm64 is an ancient CPU model, pretty much awful as qemu64. By using it you are intentionally throwing away performance that your host CPUs have. eg your host CPU almost certainly has ‘aes’ support but with kvm64 your guest won’t see it, so it will do pure software AES and so needlessly burn CPU cycles for TLS connections.
Second, guest kernels look at the CPU family to determine whether they’re running on a CPU that does speculation and thus needs mitigation. If they see kvm64 they’ll think the CPU is so old that no mitigations are needed, even if you did turn on spec-ctrl feature. So the guest OS will likely remain vulnerable.
@Lars it is complicated to fix the problem while still maintaining backwards compatibility, which is required in order to ensure live migration still works. This blog post is just a stop gap though. We’re working on ways to improve the CPU model configuration process to make it easier to get a secure config by default in future.
Is there a tool to query for what model+flags the current host-model would be? Or even better, which exists across a fleet of hypervisors?
“virsh capabilities” reports the current host model as seen by the OS. KVM will not support some features for use in guests, however, so modern libvirt also has “virsh domcapabilities kvm” which gives a more accurate answer on what “host-model” will expose to guests.
Hi Daniel, If the host OS has already turned on all the security mitigations, does the guest OS still needs to turn on those mitigations? If so, will the guest OS see “double” performance impact due to the mitigations?
It depends on which flaw you are talking about. Some require mitigating in the host only, others also require mitigating in the guest. Merely exposing the CPU flags to the guest does not force the guest OS to actually mitigate the flaws. It just gives the guest OS the ability to mitigate if it chooses to. So the flags described should always be exposed to guests, to allow the guest OS to make the decision.
Is this a complete answer ? See https://serverfault.com/a/582985 The implication seems to be that the -host option may add things like APIC support which the host can emulate.
Think of “-cpu host” as saying “Expose the best that the host OS + CPU is able to support”. The guest CPU will match host CPU as closely as possible. Some host features might still be missing as some are impossible to virtualize. There might also be a few extra things in the guest, if the hypervisor can virtualize some things that only make sense in guest context.
I am using virt-manager and select copy host cpu configuration.
OS is ubuntu 18.04 5.0.0-32 kernel and my cpu is i5-3570k (IvyBridge)
However it changes it to EPYC as soon as I click start button.
I have no idea what is going on ?
It is best to take this kind of question to the users mailing list https://libvirt.org/contact.html#email
Hello, I have to select Ivy Bridge since I have Xeon 2678 but I cannot find it on the drop-down menu in VirtManager. How to solve this? thanks a lot in advace!
virt-manager will only display CPU models that are capable of running on your host. So any CPU model that is missing is likely not compatible.