libvirt users - Feb 2017 - Why Guest does not retain Host CPU flags

hi ,

I am creating Guest by Libvirt tool " virt-install"  with cpu model as
host.

After Guest creation i see Guest Missing quite no of flags which Host was
having .

What could be the reason for this ?  I am expecting all flags of Host to e
present in Guest as well when choosing cpu model as host .

Guest :


processor    : 19
vendor_id    : GenuineIntel
cpu family    : 6
model        : 42
model name    : Intel Xeon E312xx (Sandy Bridge)
stepping    : 1
microcode    : 0x1
cpu MHz        : 1995.266
cache size    : 4096 KB
fpu        : yes
fpu_exception    : yes
cpuid level    : 13
wp        : yes
flags        : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca
cmov pat pse36 clflush mmx fxsr sse sse2 ss syscall nx pdpe1gb rdtscp lm
constant_tsc rep_good nopl eagerfpu pni pclmulqdq vmx ssse3 cx16 pcid
sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx hypervisor
lahf_lm xsaveopt tpr_shadow vnmi flexpriority ept vpid
bogomips    : 4013.27
clflush size    : 64
cache_alignment    : 64
address sizes    : 46 bits physical, 48 bits virtual
power management:

HOst :

processor    : 23
vendor_id    : GenuineIntel
cpu family    : 6
model        : 45
model name    : Intel(R) Xeon(R) CPU E5-2620 0 @ 2.00GHz
stepping    : 7
microcode    : 0x710
cpu MHz        : 1400.000
cache size    : 15360 KB
physical id    : 1
siblings    : 12
core id        : 5
cpu cores    : 6
apicid        : 43
initial apicid    : 43
fpu        : yes
fpu_exception    : yes
cpuid level    : 13
wp        : yes
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 arch_perfmon pebs bts rep_good nopl
xtopology nonstop_tsc aperfmperf eagerfpu pni pclmulqdq dtes64 monitor
ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic
popcnt tsc_deadline_timer aes xsave avx lahf_lm ida arat epb pln pts dtherm
tpr_shadow vnmi flexpriority ept vpid xsaveopt
bogomips    : 4004.01
clflush size    : 64
cache_alignment    : 64
address sizes    : 46 bits physical, 48 bits virtual
power management:


br aki

On Mon, Feb 06, 2017 at 09:51:06 +0530, akhilesh rawat
wrote:
> hi ,
> 
> I am creating Guest by Libvirt tool " virt-install"  with cpu
model as
> host.
> 
> After Guest creation i see Guest Missing quite no of flags which Host was
> having .
> 
> What could be the reason for this ?  I am expecting all flags of Host to e
> present in Guest as well when choosing cpu model as host .
Even if you request "host" CPU model, the guest will always see only a
subset of features the host supports. Not all features can be used in a
virtual machine and even if they can be used they usually need some
support form KVM or QEMU or both. That is both KVM and QEMU will filter
out features they cannot currently virtualize and thus you will get a
smaller set in comparison to the host CPU.

Jirka

ok thanks , but then can we know which all list of feature are supported by
QEMU/KVM release.
http://www.linux-kvm.org/page/KVM_Features
Does not seem to be all corect .



On Thu, Feb 16, 2017 at 3:42 PM, Jiri Denemark <jdenemar@redhat.com>
wrote:
> On Mon, Feb 06, 2017 at 09:51:06 +0530, akhilesh rawat wrote:
> > hi ,
> >
> > I am creating Guest by Libvirt tool " virt-install"  with
cpu model as
> > host.
> >
> > After Guest creation i see Guest Missing quite no of flags which Host
was
> > having .
> >
> > What could be the reason for this ?  I am expecting all flags of Host
to
> e
> > present in Guest as well when choosing cpu model as host .
>
> Even if you request "host" CPU model, the guest will always see
only a
> subset of features the host supports. Not all features can be used in a
> virtual machine and even if they can be used they usually need some
> support form KVM or QEMU or both. That is both KVM and QEMU will filter
> out features they cannot currently virtualize and thus you will get a
> smaller set in comparison to the host CPU.
>
> Jirka
>