[libvirt-users] CPU model and missing AES-NI extension

[Dennis Jacobfeuerborn]

On 05.02.2015 13:00, Michal Privoznik wrote:
> On 04.02.2015 22:32, Dennis Jacobfeuerborn wrote:
>> Hi,
>> today I tried to configure a guest using Virt-Manager and used the "copy
>> host cpu configuration" option which resultet in a "Sandy Bridge" model.
>> What I noticed is that for example the "aes" extension is not available
>> in the guest even though it is available on the host cpu.
>>
>> This is what the host cpu looks like:
>>
>> model name	: Intel(R) Xeon(R) CPU E5-2650 v3 @ 2.30GHz
>> 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 fma cx16 xtpr pdcm pcid dca sse4_1 sse4_2
>> x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm
>> abm ida arat epb xsaveopt pln pts dtherm tpr_shadow vnmi flexpriority
>> ept vpid fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid
>>
>> And this is what I get in the guest:
>>
>> model name	: Intel Xeon E312xx (Sandy Bridge)
>> 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 lm
>> constant_tsc pni ssse3 cx16 sse4_1 sse4_2 popcnt lahf_lm abm
>>
>> Is there a way to get closer to the hardware and get as many of the host
>> features in the guest as possible? I never intend to do anything like
>> live-migration with this guest so compatibility for that case is not a
>> concern.
>>
> 
> I think this is XML snippet you're aiming for:
> 
>   <cpu mode='host-model'>
>     <model fallback='allow'/>
>   </cpu>

Hm, according to the documentation mode='host-model' should not be used?

"Beware, due to the way libvirt detects host CPU and due to the fact
libvirt does not talk to QEMU/KVM when creating the CPU model, CPU
configuration created using host-model may not work as expected. The
guest CPU may differ from the configuration and it may also confuse
guest OS by using a combination of CPU features and other parameters
(such as CPUID level) that don't work. Until these issues are fixed,
it's a good idea to avoid using host-model and use custom mode with just
the CPU model from host capabilities XML."

Regards,
 Dennis