KVM : CPU host-passthrough

“CPU host-passthrough” causes libvirt to tell KVM to passthrough the host CPU with no modification

# cat /etc/fedora-release ;uname -ri;qemu-system-x86_64 --version Fedora release 19 (Schrödinger’s Cat) 3.10.6-200.fc19.x86_64 x86_64 QEMU emulator version 1.6.0, Copyright (c) 2003-2008 Fabrice Bellard

add red lines in a VM’s xml file

<vcpu placement='static'>1</vcpu>  <resource>    <partition>/machine</partition>  </resource>  <os>    <type arch='x86_64' machine='pc-i440fx-1.4'>hvm</type>    <boot dev='hd'/>  </os>  <features>    <acpi/>    <apic/>    <pae/>  </features>  <cpu mode='host-passthrough'>  </cpu>

nn, different CPU flags..

Seen from “https://wiki.openstack.org/wiki/LibvirtXMLCPUModel”, it describes that “the difference to host-model, instead of just matching feature flags, every last detail of the host CPU is matched.”

KVM host

model name : AMD Phenom(tm) II X4 925 Processor flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm 3dnowext 3dnow constant_tsc rep_good nopl nonstop_tsc extd_apicid pni monitor cx16 popcnt lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw ibs skinit wdt hw_pstate npt lbrv svm_lock nrip_save

VM

model name : AMD Phenom(tm) II X4 925 Processor flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 syscall nx mmxext fxsr_opt pdpe1gb lm 3dnowext 3dnow up rep_good extd_apicid unfair_spinlock pni cx16 x2apic popcnt hypervisor lahf_lm cmp_legacy svm cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw npt nrip_save

You can check what kind of CPU type you can specify to VMs.

# cat /usr/share/libvirt/cpu_map.xml <cpus>  <arch name='x86'>    <!-- vendor definitions -->    <vendor name='Intel' string='GenuineIntel'/>    <vendor name='AMD' string='AuthenticAMD'/>    <!-- standard features, EDX -->    <feature name='fpu'> <!-- CPUID_FP87 -->      <cpuid function='0x00000001' edx='0x00000001'/>    </feature>