[libvirt] Question on output of virsh capabilities

Jiri Denemark jdenemar at redhat.com
Thu May 26 08:58:00 UTC 2016

On Thu, May 26, 2016 at 16:44:47 +0800, Eli Qiao wrote:
> hi
> I am confused by the libvirt's "virsh capabilities"
> help said:
>      Returns capabilities of hypervisor/driver.
> I got follow outtput of a host.
>      <cpu>
>        <arch>x86_64</arch>
>        <model>Broadwell-noTSX</model>
>        <vendor>Intel</vendor>
>        <topology sockets='1' cores='2' threads='2'/>
>        <feature name='invtsc'/>
>        <feature name='mpx'/>
>        <feature name='abm'/>
>        <feature name='pdpe1gb'/>
>        <feature name='rdrand'/>
>        <feature name='f16c'/>
>        <feature name='osxsave'/>
>        <feature name='pdcm'/>
>        <feature name='xtpr'/>
>        <feature name='tm2'/>
>        <feature name='est'/>
>        <feature name='vmx'/>
>        <feature name='ds_cpl'/>
>        <feature name='monitor'/>
>        <feature name='dtes64'/>
>        <feature name='pbe'/>
>        <feature name='tm'/>
>        <feature name='ht'/>
>        <feature name='ss'/>
>        <feature name='acpi'/>
>        <feature name='ds'/>
>        <feature name='vme'/>
>        <pages unit='KiB' size='4'/>
>        <pages unit='KiB' size='2048'/>
>        <pages unit='KiB' size='1048576'/>
>      </cpu>
> but from cat /proc/cpuinfo
> flags        : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca 
> cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall 
> nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good 
> nopl xtopology nonstop_tsc aperfmperf eagerfpu pni pclmulqdq dtes64 
> monitor ds_cpl vmx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1 
> sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand 
> lahf_lm abm 3dnowprefetch epb intel_pt tpr_shadow vnmi flexpriority ept 
> vpid fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid mpx rdseed adx 
> smap clflushopt xsaveopt xsavec xgetbv1 dtherm ida arat pln pts hwp 
> hwp_notify hwp_act_window hwp_epp
> The output of virsh seems missed some of the cpu features so my question 
> what doesn't the features of "virsh capibalities" mean?
> why not pass all cpu flags?

Some of them are missing because libvirt doesn't know about them yet
(e.g, xsaveopt), but most of them are missing because they are included
in the Broadwell-noTSX CPU model. Libvirt explicitly shows only features
that are not included in the CPU model. If you really want to see all
CPUID features libvirt detected, you can pass the whole <cpu>...</cpu>
element from the capabilities XML to "virsh baseline --features"


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