libvirt cpu mode

libvirt中 cpu mode可以有以下3种：

custom :

该模式下cpu element用来描述guest可见的CPU，该模式也是mode的default模式，它会使得persistent guest不论host CPU是怎样，guest可见的CPU是一样的。有利于migration。

host-model :

该模式下，domain初始化时会copy当前host cpu的capabilities到guest 的domain。match attribute 和任何feature elements不适用该模式。

host-passthrough:

该模式下，guest可见的CPU与当前运行之上的host的CPU完全相同，model和任何feature elements不适用该模式。

https://libvirt.org/formatdomain.html#elementsCPU

https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Virtualization_Administration_Guide/sect-libvirt-dom-xml-cpu-model-top.html

libvirt cpu mode 在nova中通过一下nova.conf来配置：

# Set to "host-model" to clone the host CPU feature flags; to "host-
# passthrough" to use the host CPU model exactly; to "custom" to use a named
# CPU model; to "none" to not set any CPU model. If virt_type="kvm|qemu", it
# will default to "host-model", otherwise it will default to "none" (string
# value)
# Allowed values: host-model, host-passthrough, custom, none
#cpu_mode = <None>

 https://wiki.openstack.org/wiki/LibvirtXMLCPUModel

https://review.openstack.org/#/c/286520/3/specs/newton/approved/expose-host-capabilities.rst

打算扩展和统一host capability，主要在现有的host capability之上扩展以下3个方面：

1. 版本capability
2. 用在image filter的architecture, hypervisor type, VM mode
3. 不易被发现的e.g.: SSD, SR-IOV, fibre channel, etc。

https://review.openstack.org/#/c/309762/2/specs/newton/approved/standardize-capabilities.rst

试图采用枚举的方式(nova.objects.fields.Enum-based)来同一capability的描述模型。

它首先定义了capabilities：

A capability is a *singular* piece of information describing some **functionality** or **feature** of an entity that can be provided to a custom。

比如：

* `hw:x86_cpu_instruction_set_ext:sse`

* `hw:x86_cpu_instruction_set_ext:mmx`

用来表述CPU的指令集

* `virt:libvirt:set_admin_pass`

* `virt:libvirt:huge_pages`

* `virt:libvirt:uefi_boot`

用来表述libvirt的capability.